Related
Arrow functions in ES2015 provide a more concise syntax.
Can I replace all my function declarations / expressions with arrow functions now?
What do I have to look out for?
Examples:
Constructor function
function User(name) {
this.name = name;
}
// vs
const User = name => {
this.name = name;
};
Prototype methods
User.prototype.getName = function() {
return this.name;
};
// vs
User.prototype.getName = () => this.name;
Object (literal) methods
const obj = {
getName: function() {
// ...
}
};
// vs
const obj = {
getName: () => {
// ...
}
};
Callbacks
setTimeout(function() {
// ...
}, 500);
// vs
setTimeout(() => {
// ...
}, 500);
Variadic functions
function sum() {
let args = [].slice.call(arguments);
// ...
}
// vs
const sum = (...args) => {
// ...
};
tl;dr: No! Arrow functions and function declarations / expressions are not equivalent and cannot be replaced blindly.
If the function you want to replace does not use this, arguments and is not called with new, then yes.
As so often: it depends. Arrow functions have different behavior than function declarations / expressions, so let's have a look at the differences first:
1. Lexical this and arguments
Arrow functions don't have their own this or arguments binding. Instead, those identifiers are resolved in the lexical scope like any other variable. That means that inside an arrow function, this and arguments refer to the values of this and arguments in the environment the arrow function is defined in (i.e. "outside" the arrow function):
// Example using a function expression
function createObject() {
console.log('Inside `createObject`:', this.foo);
return {
foo: 42,
bar: function() {
console.log('Inside `bar`:', this.foo);
},
};
}
createObject.call({foo: 21}).bar(); // override `this` inside createObject
// Example using a arrow function
function createObject() {
console.log('Inside `createObject`:', this.foo);
return {
foo: 42,
bar: () => console.log('Inside `bar`:', this.foo),
};
}
createObject.call({foo: 21}).bar(); // override `this` inside createObject
In the function expression case, this refers to the object that was created inside the createObject. In the arrow function case, this refers to this of createObject itself.
This makes arrow functions useful if you need to access the this of the current environment:
// currently common pattern
var that = this;
getData(function(data) {
that.data = data;
});
// better alternative with arrow functions
getData(data => {
this.data = data;
});
Note that this also means that is not possible to set an arrow function's this with .bind or .call.
If you are not very familiar with this, consider reading
MDN - this
YDKJS - this & Object prototypes
2. Arrow functions cannot be called with new
ES2015 distinguishes between functions that are callable and functions that are constructable. If a function is constructable, it can be called with new, i.e. new User(). If a function is callable, it can be called without new (i.e. normal function call).
Functions created through function declarations / expressions are both constructable and callable.
Arrow functions (and methods) are only callable.
class constructors are only constructable.
If you are trying to call a non-callable function or to construct a non-constructable function, you will get a runtime error.
Knowing this, we can state the following.
Replaceable:
Functions that don't use this or arguments.
Functions that are used with .bind(this)
Not replaceable:
Constructor functions
Function / methods added to a prototype (because they usually use this)
Variadic functions (if they use arguments (see below))
Generator functions, which require the function* notation
Lets have a closer look at this using your examples:
Constructor function
This won't work because arrow functions cannot be called with new. Keep using a function declaration / expression or use class.
Prototype methods
Most likely not, because prototype methods usually use this to access the instance. If they don't use this, then you can replace it. However, if you primarily care for concise syntax, use class with its concise method syntax:
class User {
constructor(name) {
this.name = name;
}
getName() {
return this.name;
}
}
Object methods
Similarly for methods in an object literal. If the method wants to reference the object itself via this, keep using function expressions, or use the new method syntax:
const obj = {
getName() {
// ...
},
};
Callbacks
It depends. You should definitely replace it if you are aliasing the outer this or are using .bind(this):
// old
setTimeout(function() {
// ...
}.bind(this), 500);
// new
setTimeout(() => {
// ...
}, 500);
But: If the code which calls the callback explicitly sets this to a specific value, as is often the case with event handlers, especially with jQuery, and the callback uses this (or arguments), you cannot use an arrow function!
Variadic functions
Since arrow functions don't have their own arguments, you cannot simply replace them with an arrow function. However, ES2015 introduces an alternative to using arguments: the rest parameter.
// old
function sum() {
let args = [].slice.call(arguments);
// ...
}
// new
const sum = (...args) => {
// ...
};
Related question:
When should I use arrow functions in ECMAScript 6?
Do ES6 arrow functions have their own arguments or not?
What are the differences (if any) between ES6 arrow functions and functions bound with Function.prototype.bind?
How to use arrow functions (public class fields) as class methods?
Further resources:
MDN - Arrow functions
YDKJS - Arrow functions
Arrow functions => best ES6 feature so far. They are a tremendously
powerful addition to ES6, that I use constantly.
Wait, you can't use arrow function everywhere in your code, its not going to work in all cases like this where arrow functions are not usable. Without a doubt, the arrow function is a great addition it brings code simplicity.
But you can’t use an arrow function when a dynamic context is required: defining methods, create objects with constructors, get the target from this when handling events.
Arrow functions should NOT be used because:
They do not have this
It uses “lexical scoping” to figure out what the value of “this”
should be. In simple word lexical scoping it uses “this” from the
inside the function’s body.
They do not have arguments
Arrow functions don’t have an arguments object. But the same
functionality can be achieved using rest parameters.
let sum = (...args) => args.reduce((x, y) => x + y, 0);
sum(3, 3, 1) // output: 7
They cannot be used with new
Arrow functions can't be constructors because they do not have a prototype property.
When to use arrow function and when not:
Don't use to add function as a property in object literal because we
can not access this.
Function expressions are best for object methods. Arrow functions
are best for callbacks or methods like map, reduce, or forEach.
Use function declarations for functions you’d call by name (because
they’re hoisted).
Use arrow functions for callbacks (because they tend to be terser).
To use arrow functions with function.prototype.call, I made a helper function on the object prototype:
// Using
// #func = function() {use this here} or This => {use This here}
using(func) {
return func.call(this, this);
}
usage
var obj = {f:3, a:2}
.using(This => This.f + This.a) // 5
Edit
You don't NEED a helper. You could do:
var obj = {f:3, a:2}
(This => This.f + This.a).call(undefined, obj); // 5
They are not always equivalent. Here's a case where you cannot simply use arrow functions instead of regular functions.
Arrow functions CANNOT be used as constructors
TLDR:
This is because of how Arrow Functions use the this keyword. JS will simply throw an error if it sees an arrow function being invoked as a "constructor". Use regular functions to fix the error.
Longer explanation:
This is because objects "constructors" rely on the this keyword to be able to be modified.
Generally, the this keyword always references the global object. (In the browser it is the window object).
BUT, when you do something like:
function personCreator(name) {
this.name = name;
}
const person1 = new personCreator('John');
The new keyword do some of its magic and makes the this keyword that is inside of personCreator to be initially an empty object instead of referencing the global object. After that, a new property called name is created inside that empty this object, and its value will be 'John'. At the end, the this object is returned.
As we see, the new keyword changed the value of this from referencing the global object to now be an empty object {}.
Arrow functions do not allow their this object to be modified. Their this object is always the one from the scope where they were statically created. This is called Static Lexical Scope. That is why you cannot do operations like bind, apply, or call with arrow functions. Simply, their this is locked to the value of the this of the scope were they were created. This is by design.
And because of this :D, arrow functions cannot be used as "constructors".
Side Note:
A lexical scope is just the area where a function is created. For example:
function personCreator(name) {
this.name = name;
const foo = () => {
const bar = () => {
console.log(this); // Output: { name: 'John' }
}
console.log(this); // Output: { name: 'John' }
bar();
}
foo();
}
const person1 = new personCreator('John');
The lexical scope of bar is everything that is within foo. So, the this value of bar is the one that foo has, which is the one of personCreator.
I have a constructor function which registers an event handler:
function MyConstructor(data, transport) {
this.data = data;
transport.on('data', function () {
alert(this.data);
});
}
// Mock transport object
var transport = {
on: function(event, callback) {
setTimeout(callback, 1000);
}
};
// called as
var obj = new MyConstructor('foo', transport);
However, I'm not able to access the data property of the created object inside the callback. It looks like this does not refer to the object that was created, but to another one.
I also tried to use an object method instead of an anonymous function:
function MyConstructor(data, transport) {
this.data = data;
transport.on('data', this.alert);
}
MyConstructor.prototype.alert = function() {
alert(this.name);
};
but it exhibits the same problems.
How can I access the correct object?
What you should know about this
this (aka "the context") is a special keyword inside each function and its value only depends on how the function was called, not how/when/where it was defined. It is not affected by lexical scopes like other variables (except for arrow functions, see below). Here are some examples:
function foo() {
console.log(this);
}
// normal function call
foo(); // `this` will refer to `window`
// as object method
var obj = {bar: foo};
obj.bar(); // `this` will refer to `obj`
// as constructor function
new foo(); // `this` will refer to an object that inherits from `foo.prototype`
To learn more about this, have a look at the MDN documentation.
How to refer to the correct this
Use arrow functions
ECMAScript 6 introduced arrow functions, which can be thought of as lambda functions. They don't have their own this binding. Instead, this is looked up in scope just like a normal variable. That means you don't have to call .bind. That's not the only special behavior they have, please refer to the MDN documentation for more information.
function MyConstructor(data, transport) {
this.data = data;
transport.on('data', () => alert(this.data));
}
Don't use this
You actually don't want to access this in particular, but the object it refers to. That's why an easy solution is to simply create a new variable that also refers to that object. The variable can have any name, but common ones are self and that.
function MyConstructor(data, transport) {
this.data = data;
var self = this;
transport.on('data', function() {
alert(self.data);
});
}
Since self is a normal variable, it obeys lexical scope rules and is accessible inside the callback. This also has the advantage that you can access the this value of the callback itself.
Explicitly set this of the callback - part 1
It might look like you have no control over the value of this because its value is set automatically, but that is actually not the case.
Every function has the method .bind [docs], which returns a new function with this bound to a value. The function has exactly the same behavior as the one you called .bind on, only that this was set by you. No matter how or when that function is called, this will always refer to the passed value.
function MyConstructor(data, transport) {
this.data = data;
var boundFunction = (function() { // parenthesis are not necessary
alert(this.data); // but might improve readability
}).bind(this); // <- here we are calling `.bind()`
transport.on('data', boundFunction);
}
In this case, we are binding the callback's this to the value of MyConstructor's this.
Note: When a binding context for jQuery, use jQuery.proxy [docs] instead. The reason to do this is so that you don't need to store the reference to the function when unbinding an event callback. jQuery handles that internally.
Set this of the callback - part 2
Some functions/methods which accept callbacks also accept a value to which the callback's this should refer to. This is basically the same as binding it yourself, but the function/method does it for you. Array#map [docs] is such a method. Its signature is:
array.map(callback[, thisArg])
The first argument is the callback and the second argument is the value this should refer to. Here is a contrived example:
var arr = [1, 2, 3];
var obj = {multiplier: 42};
var new_arr = arr.map(function(v) {
return v * this.multiplier;
}, obj); // <- here we are passing `obj` as second argument
Note: Whether or not you can pass a value for this is usually mentioned in the documentation of that function/method. For example, jQuery's $.ajax method [docs] describes an option called context:
This object will be made the context of all Ajax-related callbacks.
Common problem: Using object methods as callbacks/event handlers
Another common manifestation of this problem is when an object method is used as callback/event handler. Functions are first-class citizens in JavaScript and the term "method" is just a colloquial term for a function that is a value of an object property. But that function doesn't have a specific link to its "containing" object.
Consider the following example:
function Foo() {
this.data = 42,
document.body.onclick = this.method;
}
Foo.prototype.method = function() {
console.log(this.data);
};
The function this.method is assigned as click event handler, but if the document.body is clicked, the value logged will be undefined, because inside the event handler, this refers to the document.body, not the instance of Foo.
As already mentioned at the beginning, what this refers to depends on how the function is called, not how it is defined.
If the code was like the following, it might be more obvious that the function doesn't have an implicit reference to the object:
function method() {
console.log(this.data);
}
function Foo() {
this.data = 42,
document.body.onclick = this.method;
}
Foo.prototype.method = method;
The solution is the same as mentioned above: If available, use .bind to explicitly bind this to a specific value
document.body.onclick = this.method.bind(this);
or explicitly call the function as a "method" of the object, by using an anonymous function as callback / event handler and assign the object (this) to another variable:
var self = this;
document.body.onclick = function() {
self.method();
};
or use an arrow function:
document.body.onclick = () => this.method();
Here are several ways to access the parent context inside a child context -
You can use the bind() function.
Store a reference to context/this inside another variable (see the below example).
Use ES6 Arrow functions.
Alter the code, function design, and architecture - for this you should have command over design patterns in JavaScript.
1. Use the bind() function
function MyConstructor(data, transport) {
this.data = data;
transport.on('data', ( function () {
alert(this.data);
}).bind(this) );
}
// Mock transport object
var transport = {
on: function(event, callback) {
setTimeout(callback, 1000);
}
};
// called as
var obj = new MyConstructor('foo', transport);
If you are using Underscore.js - http://underscorejs.org/#bind
transport.on('data', _.bind(function () {
alert(this.data);
}, this));
2. Store a reference to context/this inside another variable
function MyConstructor(data, transport) {
var self = this;
this.data = data;
transport.on('data', function() {
alert(self.data);
});
}
3. Arrow function
function MyConstructor(data, transport) {
this.data = data;
transport.on('data', () => {
alert(this.data);
});
}
It's all in the "magic" syntax of calling a method:
object.property();
When you get the property from the object and call it in one go, the object will be the context for the method. If you call the same method, but in separate steps, the context is the global scope (window) instead:
var f = object.property;
f();
When you get the reference of a method, it's no longer attached to the object. It's just a reference to a plain function. The same happens when you get the reference to use as a callback:
this.saveNextLevelData(this.setAll);
That's where you would bind the context to the function:
this.saveNextLevelData(this.setAll.bind(this));
If you are using jQuery you should use the $.proxy method instead, as bind is not supported in all browsers:
this.saveNextLevelData($.proxy(this.setAll, this));
You should know about "this" Keyword.
As per my view you can implement "this" in three ways
(Self|Arrow function|Bind Method)
A function's this keyword behaves a little differently in JavaScript compared to other languages.
It also has some differences between strict mode and non-strict mode.
In most cases, the value of this is determined by how a function is called.
It can't be set by assignment during execution, and it may be different each time the function is called.
ES5 introduced the bind() method to set the value of a function's this regardless of how it's called,
And ES2015 introduced arrow functions that don't provide their own this binding (it retains this value of the enclosing lexical context).
Method1: Self - Self is being used to maintain a reference to the original this even as the context is changing. It's a technique often used in event handlers (especially in closures).
Reference: this
function MyConstructor(data, transport) {
this.data = data;
var self = this;
transport.on('data', function () {
alert(self.data);
});
}
Method2: Arrow function - An arrow function expression is a syntactically compact alternative to a regular function expression, although without its own bindings to the this, arguments, super, or new.target keywords.
Arrow function expressions are ill-suited as methods, and they cannot be used as constructors.
Reference: Arrow function expressions
function MyConstructor(data, transport) {
this.data = data;
transport.on('data',()=> {
alert(this.data);
});
}
Method 3: Bind - The bind() method creates a new function that, when called, has its this keyword set to the provided value with a given sequence of arguments preceding any provided when the new function is called.
Reference: Function.prototype.bind()
function MyConstructor(data, transport) {
this.data = data;
transport.on('data',(function() {
alert(this.data);
}).bind(this);
The trouble with "context"
The term "context" is sometimes used to refer to the object referenced by this. Its use is inappropriate, because it doesn't fit either semantically or technically with ECMAScript's this.
"Context" means the circumstances surrounding something that adds meaning, or some preceding and following information that gives extra meaning. The term "context" is used in ECMAScript to refer to execution context, which is all the parameters, scope, and this within the scope of some executing code.
This is shown in ECMA-262 section 10.4.2:
Set the ThisBinding to the same value as the ThisBinding of the
calling execution context
Which clearly indicates that this is part of an execution context.
An execution context provides the surrounding information that adds meaning to the code that is being executed. It includes much more information than just the thisBinding.
The value of this isn't "context". It's just one part of an execution context. It's essentially a local variable that can be set by the call to any object and in strict mode, to any value at all.
First, you need to have a clear understanding of scope and behaviour of the this keyword in the context of scope.
this & scope:
There are two types of scope in JavaScript. They are:
Global Scope
Function Scope
In short, global scope refers to the window object. Variables declared in a global scope are accessible from anywhere.
On the other hand, function scope resides inside of a function. A variable declared inside a function cannot be accessed from the outside world normally.
The this keyword in the global scope refers to the window object. this inside a function also refers to the window object. So this will always refer to the window until we find a way to manipulate this to indicate a context of our own choosing.
--------------------------------------------------------------------------------
- -
- Global Scope -
- (globally "this" refers to window object) -
- -
- function outer_function(callback){ -
- -
- // Outer function scope -
- // Inside the outer function, the "this" keyword -
- // refers to window object -
- callback() // "this" inside callback also refers to the window object -
- } -
- -
- function callback_function(){ -
- -
- // Function to be passed as callback -
- -
- // Here "THIS" refers to the window object also -
- } -
- -
- outer_function(callback_function) -
- // Invoke with callback -
- -
--------------------------------------------------------------------------------
Different ways to manipulate this inside callback functions:
Here I have a constructor function called Person. It has a property called name and four method called sayNameVersion1, sayNameVersion2, sayNameVersion3, and sayNameVersion4. All four of them has one specific task. Accept a callback and invoke it. The callback has a specific task which is to log the name property of an instance of Person constructor function.
function Person(name){
this.name = name
this.sayNameVersion1 = function(callback){
callback.bind(this)()
}
this.sayNameVersion2 = function(callback){
callback()
}
this.sayNameVersion3 = function(callback){
callback.call(this)
}
this.sayNameVersion4 = function(callback){
callback.apply(this)
}
}
function niceCallback(){
// Function to be used as callback
var parentObject = this
console.log(parentObject)
}
Now let's create an instance from person constructor and invoke different versions of sayNameVersionX (X refers to 1,2,3,4) method with niceCallback to see how many ways we can manipulate the this inside callback to refer to the person instance.
var p1 = new Person('zami') // Create an instance of Person constructor
bind:
What bind do is to create a new function with the this keyword set to the provided value.
sayNameVersion1 and sayNameVersion2 use bind to manipulate this of the callback function.
this.sayNameVersion1 = function(callback){
callback.bind(this)()
}
this.sayNameVersion2 = function(callback){
callback()
}
The first one binds this with a callback inside the method itself. And for the second one, the callback is passed with the object bound to it.
p1.sayNameVersion1(niceCallback) // pass simply the callback and bind happens inside the sayNameVersion1 method
p1.sayNameVersion2(niceCallback.bind(p1)) // uses bind before passing callback
call:
The first argument of the call method is used as this inside the function that is invoked with call attached to it.
sayNameVersion3 uses call to manipulate the this to refer to the person object that we created, instead of the window object.
this.sayNameVersion3 = function(callback){
callback.call(this)
}
And it is called like the following:
p1.sayNameVersion3(niceCallback)
apply:
Similar to call, the first argument of apply refers to the object that will be indicated by the this keyword.
sayNameVersion4 uses apply to manipulate this to refer to a person object
this.sayNameVersion4 = function(callback){
callback.apply(this)
}
And it is called like the following. Simply the callback is passed,
p1.sayNameVersion4(niceCallback)
We can not bind this to setTimeout(), as it always executes with the global object (Window). If you want to access the this context in the callback function then by using bind() to the callback function, we can achieve it as:
setTimeout(function(){
this.methodName();
}.bind(this), 2000);
The question revolves around how the this keyword behaves in JavaScript. this behaves differently as below,
The value of this is usually determined by a function execution context.
In the global scope, this refers to the global object (the window object).
If strict mode is enabled for any function then the value of this will be undefined as in strict mode, global object refers to undefined in place of the window object.
The object that is standing before the dot is what the this keyword will be bound to.
We can set the value of this explicitly with call(), bind(), and apply()
When the new keyword is used (a constructor), this is bound to the new object being created.
Arrow functions don’t bind this — instead, this is bound lexically (i.e., based on the original context)
As most of the answers suggest, we can use the arrow function or bind() Method or Self var. I would quote a point about lambdas (arrow function) from Google JavaScript Style Guide
Prefer using arrow functions over f.bind(this), and especially over
goog.bind(f, this). Avoid writing const self = this. Arrow functions
are particularly useful for callbacks, which sometimes pass unexpectedly
additional arguments.
Google clearly recommends using lambdas rather than bind or const self = this
So the best solution would be to use lambdas as below,
function MyConstructor(data, transport) {
this.data = data;
transport.on('data', () => {
alert(this.data);
});
}
References:
https://medium.com/tech-tajawal/javascript-this-4-rules-7354abdb274c
arrow-functions-vs-bind
Currently there is another approach possible if classes are used in code.
With support of class fields, it's possible to make it the following way:
class someView {
onSomeInputKeyUp = (event) => {
console.log(this); // This refers to the correct value
// ....
someInitMethod() {
//...
someInput.addEventListener('input', this.onSomeInputKeyUp)
For sure under the hood it's all the old good arrow function that binds context, but in this form it looks much more clear that explicit binding.
Since it's a Stage 3 Proposal, you will need Babel and appropriate Babel plugin to process it as for now (08/2018).
I was facing a problem with Ngx line chart xAxisTickFormatting function which was called from HTML like this: [xAxisTickFormatting]="xFormat".
I was unable to access my component's variable from the function declared. This solution helped me to resolve the issue to find the correct this.
Instead of using the function like this:
xFormat (value): string {
return value.toString() + this.oneComponentVariable; //gives wrong result
}
Use this:
xFormat = (value) => {
// console.log(this);
// now you have access to your component variables
return value + this.oneComponentVariable
}
Another approach, which is the standard way since DOM2 to bind this within the event listener, that let you always remove the listener (among other benefits), is the handleEvent(evt) method from the EventListener interface:
var obj = {
handleEvent(e) {
// always true
console.log(this === obj);
}
};
document.body.addEventListener('click', obj);
Detailed information about using handleEvent can be found here: DOM handleEvent: a cross-platform standard since year 2000
Some other people have touched on how to use the .bind() method, but specifically here is how you can use it with .then() if anyone is having trouble getting them to work together:
someFunction()
.then(function(response) {
//'this' wasn't accessible here before but now it is
}.bind(this))
As mentioned in the comments, an alternative would be to use an arrow function that doesn't have its own 'this' value
someFunction()
.then((response)=>{
//'this' was always accessible here
})
this in JavaScript:
The value of this in JavaScript is 100% determined by how a function is called, and not how it is defined. We can relatively easily find the value of this by the 'left of the dot rule':
When the function is created using the function keyword the value of this is the object left of the dot of the function which is called
If there is no object left of the dot then the value of this inside a function is often the global object (global in Node.js and window in a browser). I wouldn't recommend using the this keyword here because it is less explicit than using something like window!
There exist certain constructs like arrow functions and functions created using the Function.prototype.bind() a function that can fix the value of this. These are exceptions of the rule, but they are really helpful to fix the value of this.
Example in Node.js
module.exports.data = 'module data';
// This outside a function in node refers to module.exports object
console.log(this);
const obj1 = {
data: "obj1 data",
met1: function () {
console.log(this.data);
},
met2: () => {
console.log(this.data);
},
};
const obj2 = {
data: "obj2 data",
test1: function () {
console.log(this.data);
},
test2: function () {
console.log(this.data);
}.bind(obj1),
test3: obj1.met1,
test4: obj1.met2,
};
obj2.test1();
obj2.test2();
obj2.test3();
obj2.test4();
obj1.met1.call(obj2);
Output:
Let me walk you through the outputs one by one (ignoring the first log starting from the second):
this is obj2 because of the left of the dot rule, we can see how test1 is called obj2.test1();. obj2 is left of the dot and thus the this value.
Even though obj2 is left of the dot, test2 is bound to obj1 via the bind() method. The this value is obj1.
obj2 is left of the dot from the function which is called: obj2.test3(). Therefore obj2 will be the value of this.
In this case: obj2.test4() obj2 is left of the dot. However, arrow functions don't have their own this binding. Therefore it will bind to the this value of the outer scope which is the module.exports an object which was logged in the beginning.
We can also specify the value of this by using the call function. Here we can pass in the desired this value as an argument, which is obj2 in this case.
I have a constructor function which registers an event handler:
function MyConstructor(data, transport) {
this.data = data;
transport.on('data', function () {
alert(this.data);
});
}
// Mock transport object
var transport = {
on: function(event, callback) {
setTimeout(callback, 1000);
}
};
// called as
var obj = new MyConstructor('foo', transport);
However, I'm not able to access the data property of the created object inside the callback. It looks like this does not refer to the object that was created, but to another one.
I also tried to use an object method instead of an anonymous function:
function MyConstructor(data, transport) {
this.data = data;
transport.on('data', this.alert);
}
MyConstructor.prototype.alert = function() {
alert(this.name);
};
but it exhibits the same problems.
How can I access the correct object?
What you should know about this
this (aka "the context") is a special keyword inside each function and its value only depends on how the function was called, not how/when/where it was defined. It is not affected by lexical scopes like other variables (except for arrow functions, see below). Here are some examples:
function foo() {
console.log(this);
}
// normal function call
foo(); // `this` will refer to `window`
// as object method
var obj = {bar: foo};
obj.bar(); // `this` will refer to `obj`
// as constructor function
new foo(); // `this` will refer to an object that inherits from `foo.prototype`
To learn more about this, have a look at the MDN documentation.
How to refer to the correct this
Use arrow functions
ECMAScript 6 introduced arrow functions, which can be thought of as lambda functions. They don't have their own this binding. Instead, this is looked up in scope just like a normal variable. That means you don't have to call .bind. That's not the only special behavior they have, please refer to the MDN documentation for more information.
function MyConstructor(data, transport) {
this.data = data;
transport.on('data', () => alert(this.data));
}
Don't use this
You actually don't want to access this in particular, but the object it refers to. That's why an easy solution is to simply create a new variable that also refers to that object. The variable can have any name, but common ones are self and that.
function MyConstructor(data, transport) {
this.data = data;
var self = this;
transport.on('data', function() {
alert(self.data);
});
}
Since self is a normal variable, it obeys lexical scope rules and is accessible inside the callback. This also has the advantage that you can access the this value of the callback itself.
Explicitly set this of the callback - part 1
It might look like you have no control over the value of this because its value is set automatically, but that is actually not the case.
Every function has the method .bind [docs], which returns a new function with this bound to a value. The function has exactly the same behavior as the one you called .bind on, only that this was set by you. No matter how or when that function is called, this will always refer to the passed value.
function MyConstructor(data, transport) {
this.data = data;
var boundFunction = (function() { // parenthesis are not necessary
alert(this.data); // but might improve readability
}).bind(this); // <- here we are calling `.bind()`
transport.on('data', boundFunction);
}
In this case, we are binding the callback's this to the value of MyConstructor's this.
Note: When a binding context for jQuery, use jQuery.proxy [docs] instead. The reason to do this is so that you don't need to store the reference to the function when unbinding an event callback. jQuery handles that internally.
Set this of the callback - part 2
Some functions/methods which accept callbacks also accept a value to which the callback's this should refer to. This is basically the same as binding it yourself, but the function/method does it for you. Array#map [docs] is such a method. Its signature is:
array.map(callback[, thisArg])
The first argument is the callback and the second argument is the value this should refer to. Here is a contrived example:
var arr = [1, 2, 3];
var obj = {multiplier: 42};
var new_arr = arr.map(function(v) {
return v * this.multiplier;
}, obj); // <- here we are passing `obj` as second argument
Note: Whether or not you can pass a value for this is usually mentioned in the documentation of that function/method. For example, jQuery's $.ajax method [docs] describes an option called context:
This object will be made the context of all Ajax-related callbacks.
Common problem: Using object methods as callbacks/event handlers
Another common manifestation of this problem is when an object method is used as callback/event handler. Functions are first-class citizens in JavaScript and the term "method" is just a colloquial term for a function that is a value of an object property. But that function doesn't have a specific link to its "containing" object.
Consider the following example:
function Foo() {
this.data = 42,
document.body.onclick = this.method;
}
Foo.prototype.method = function() {
console.log(this.data);
};
The function this.method is assigned as click event handler, but if the document.body is clicked, the value logged will be undefined, because inside the event handler, this refers to the document.body, not the instance of Foo.
As already mentioned at the beginning, what this refers to depends on how the function is called, not how it is defined.
If the code was like the following, it might be more obvious that the function doesn't have an implicit reference to the object:
function method() {
console.log(this.data);
}
function Foo() {
this.data = 42,
document.body.onclick = this.method;
}
Foo.prototype.method = method;
The solution is the same as mentioned above: If available, use .bind to explicitly bind this to a specific value
document.body.onclick = this.method.bind(this);
or explicitly call the function as a "method" of the object, by using an anonymous function as callback / event handler and assign the object (this) to another variable:
var self = this;
document.body.onclick = function() {
self.method();
};
or use an arrow function:
document.body.onclick = () => this.method();
Here are several ways to access the parent context inside a child context -
You can use the bind() function.
Store a reference to context/this inside another variable (see the below example).
Use ES6 Arrow functions.
Alter the code, function design, and architecture - for this you should have command over design patterns in JavaScript.
1. Use the bind() function
function MyConstructor(data, transport) {
this.data = data;
transport.on('data', ( function () {
alert(this.data);
}).bind(this) );
}
// Mock transport object
var transport = {
on: function(event, callback) {
setTimeout(callback, 1000);
}
};
// called as
var obj = new MyConstructor('foo', transport);
If you are using Underscore.js - http://underscorejs.org/#bind
transport.on('data', _.bind(function () {
alert(this.data);
}, this));
2. Store a reference to context/this inside another variable
function MyConstructor(data, transport) {
var self = this;
this.data = data;
transport.on('data', function() {
alert(self.data);
});
}
3. Arrow function
function MyConstructor(data, transport) {
this.data = data;
transport.on('data', () => {
alert(this.data);
});
}
It's all in the "magic" syntax of calling a method:
object.property();
When you get the property from the object and call it in one go, the object will be the context for the method. If you call the same method, but in separate steps, the context is the global scope (window) instead:
var f = object.property;
f();
When you get the reference of a method, it's no longer attached to the object. It's just a reference to a plain function. The same happens when you get the reference to use as a callback:
this.saveNextLevelData(this.setAll);
That's where you would bind the context to the function:
this.saveNextLevelData(this.setAll.bind(this));
If you are using jQuery you should use the $.proxy method instead, as bind is not supported in all browsers:
this.saveNextLevelData($.proxy(this.setAll, this));
You should know about "this" Keyword.
As per my view you can implement "this" in three ways
(Self|Arrow function|Bind Method)
A function's this keyword behaves a little differently in JavaScript compared to other languages.
It also has some differences between strict mode and non-strict mode.
In most cases, the value of this is determined by how a function is called.
It can't be set by assignment during execution, and it may be different each time the function is called.
ES5 introduced the bind() method to set the value of a function's this regardless of how it's called,
And ES2015 introduced arrow functions that don't provide their own this binding (it retains this value of the enclosing lexical context).
Method1: Self - Self is being used to maintain a reference to the original this even as the context is changing. It's a technique often used in event handlers (especially in closures).
Reference: this
function MyConstructor(data, transport) {
this.data = data;
var self = this;
transport.on('data', function () {
alert(self.data);
});
}
Method2: Arrow function - An arrow function expression is a syntactically compact alternative to a regular function expression, although without its own bindings to the this, arguments, super, or new.target keywords.
Arrow function expressions are ill-suited as methods, and they cannot be used as constructors.
Reference: Arrow function expressions
function MyConstructor(data, transport) {
this.data = data;
transport.on('data',()=> {
alert(this.data);
});
}
Method 3: Bind - The bind() method creates a new function that, when called, has its this keyword set to the provided value with a given sequence of arguments preceding any provided when the new function is called.
Reference: Function.prototype.bind()
function MyConstructor(data, transport) {
this.data = data;
transport.on('data',(function() {
alert(this.data);
}).bind(this);
The trouble with "context"
The term "context" is sometimes used to refer to the object referenced by this. Its use is inappropriate, because it doesn't fit either semantically or technically with ECMAScript's this.
"Context" means the circumstances surrounding something that adds meaning, or some preceding and following information that gives extra meaning. The term "context" is used in ECMAScript to refer to execution context, which is all the parameters, scope, and this within the scope of some executing code.
This is shown in ECMA-262 section 10.4.2:
Set the ThisBinding to the same value as the ThisBinding of the
calling execution context
Which clearly indicates that this is part of an execution context.
An execution context provides the surrounding information that adds meaning to the code that is being executed. It includes much more information than just the thisBinding.
The value of this isn't "context". It's just one part of an execution context. It's essentially a local variable that can be set by the call to any object and in strict mode, to any value at all.
First, you need to have a clear understanding of scope and behaviour of the this keyword in the context of scope.
this & scope:
There are two types of scope in JavaScript. They are:
Global Scope
Function Scope
In short, global scope refers to the window object. Variables declared in a global scope are accessible from anywhere.
On the other hand, function scope resides inside of a function. A variable declared inside a function cannot be accessed from the outside world normally.
The this keyword in the global scope refers to the window object. this inside a function also refers to the window object. So this will always refer to the window until we find a way to manipulate this to indicate a context of our own choosing.
--------------------------------------------------------------------------------
- -
- Global Scope -
- (globally "this" refers to window object) -
- -
- function outer_function(callback){ -
- -
- // Outer function scope -
- // Inside the outer function, the "this" keyword -
- // refers to window object -
- callback() // "this" inside callback also refers to the window object -
- } -
- -
- function callback_function(){ -
- -
- // Function to be passed as callback -
- -
- // Here "THIS" refers to the window object also -
- } -
- -
- outer_function(callback_function) -
- // Invoke with callback -
- -
--------------------------------------------------------------------------------
Different ways to manipulate this inside callback functions:
Here I have a constructor function called Person. It has a property called name and four method called sayNameVersion1, sayNameVersion2, sayNameVersion3, and sayNameVersion4. All four of them has one specific task. Accept a callback and invoke it. The callback has a specific task which is to log the name property of an instance of Person constructor function.
function Person(name){
this.name = name
this.sayNameVersion1 = function(callback){
callback.bind(this)()
}
this.sayNameVersion2 = function(callback){
callback()
}
this.sayNameVersion3 = function(callback){
callback.call(this)
}
this.sayNameVersion4 = function(callback){
callback.apply(this)
}
}
function niceCallback(){
// Function to be used as callback
var parentObject = this
console.log(parentObject)
}
Now let's create an instance from person constructor and invoke different versions of sayNameVersionX (X refers to 1,2,3,4) method with niceCallback to see how many ways we can manipulate the this inside callback to refer to the person instance.
var p1 = new Person('zami') // Create an instance of Person constructor
bind:
What bind do is to create a new function with the this keyword set to the provided value.
sayNameVersion1 and sayNameVersion2 use bind to manipulate this of the callback function.
this.sayNameVersion1 = function(callback){
callback.bind(this)()
}
this.sayNameVersion2 = function(callback){
callback()
}
The first one binds this with a callback inside the method itself. And for the second one, the callback is passed with the object bound to it.
p1.sayNameVersion1(niceCallback) // pass simply the callback and bind happens inside the sayNameVersion1 method
p1.sayNameVersion2(niceCallback.bind(p1)) // uses bind before passing callback
call:
The first argument of the call method is used as this inside the function that is invoked with call attached to it.
sayNameVersion3 uses call to manipulate the this to refer to the person object that we created, instead of the window object.
this.sayNameVersion3 = function(callback){
callback.call(this)
}
And it is called like the following:
p1.sayNameVersion3(niceCallback)
apply:
Similar to call, the first argument of apply refers to the object that will be indicated by the this keyword.
sayNameVersion4 uses apply to manipulate this to refer to a person object
this.sayNameVersion4 = function(callback){
callback.apply(this)
}
And it is called like the following. Simply the callback is passed,
p1.sayNameVersion4(niceCallback)
We can not bind this to setTimeout(), as it always executes with the global object (Window). If you want to access the this context in the callback function then by using bind() to the callback function, we can achieve it as:
setTimeout(function(){
this.methodName();
}.bind(this), 2000);
The question revolves around how the this keyword behaves in JavaScript. this behaves differently as below,
The value of this is usually determined by a function execution context.
In the global scope, this refers to the global object (the window object).
If strict mode is enabled for any function then the value of this will be undefined as in strict mode, global object refers to undefined in place of the window object.
The object that is standing before the dot is what the this keyword will be bound to.
We can set the value of this explicitly with call(), bind(), and apply()
When the new keyword is used (a constructor), this is bound to the new object being created.
Arrow functions don’t bind this — instead, this is bound lexically (i.e., based on the original context)
As most of the answers suggest, we can use the arrow function or bind() Method or Self var. I would quote a point about lambdas (arrow function) from Google JavaScript Style Guide
Prefer using arrow functions over f.bind(this), and especially over
goog.bind(f, this). Avoid writing const self = this. Arrow functions
are particularly useful for callbacks, which sometimes pass unexpectedly
additional arguments.
Google clearly recommends using lambdas rather than bind or const self = this
So the best solution would be to use lambdas as below,
function MyConstructor(data, transport) {
this.data = data;
transport.on('data', () => {
alert(this.data);
});
}
References:
https://medium.com/tech-tajawal/javascript-this-4-rules-7354abdb274c
arrow-functions-vs-bind
Currently there is another approach possible if classes are used in code.
With support of class fields, it's possible to make it the following way:
class someView {
onSomeInputKeyUp = (event) => {
console.log(this); // This refers to the correct value
// ....
someInitMethod() {
//...
someInput.addEventListener('input', this.onSomeInputKeyUp)
For sure under the hood it's all the old good arrow function that binds context, but in this form it looks much more clear that explicit binding.
Since it's a Stage 3 Proposal, you will need Babel and appropriate Babel plugin to process it as for now (08/2018).
I was facing a problem with Ngx line chart xAxisTickFormatting function which was called from HTML like this: [xAxisTickFormatting]="xFormat".
I was unable to access my component's variable from the function declared. This solution helped me to resolve the issue to find the correct this.
Instead of using the function like this:
xFormat (value): string {
return value.toString() + this.oneComponentVariable; //gives wrong result
}
Use this:
xFormat = (value) => {
// console.log(this);
// now you have access to your component variables
return value + this.oneComponentVariable
}
Another approach, which is the standard way since DOM2 to bind this within the event listener, that let you always remove the listener (among other benefits), is the handleEvent(evt) method from the EventListener interface:
var obj = {
handleEvent(e) {
// always true
console.log(this === obj);
}
};
document.body.addEventListener('click', obj);
Detailed information about using handleEvent can be found here: DOM handleEvent: a cross-platform standard since year 2000
Some other people have touched on how to use the .bind() method, but specifically here is how you can use it with .then() if anyone is having trouble getting them to work together:
someFunction()
.then(function(response) {
//'this' wasn't accessible here before but now it is
}.bind(this))
As mentioned in the comments, an alternative would be to use an arrow function that doesn't have its own 'this' value
someFunction()
.then((response)=>{
//'this' was always accessible here
})
this in JavaScript:
The value of this in JavaScript is 100% determined by how a function is called, and not how it is defined. We can relatively easily find the value of this by the 'left of the dot rule':
When the function is created using the function keyword the value of this is the object left of the dot of the function which is called
If there is no object left of the dot then the value of this inside a function is often the global object (global in Node.js and window in a browser). I wouldn't recommend using the this keyword here because it is less explicit than using something like window!
There exist certain constructs like arrow functions and functions created using the Function.prototype.bind() a function that can fix the value of this. These are exceptions of the rule, but they are really helpful to fix the value of this.
Example in Node.js
module.exports.data = 'module data';
// This outside a function in node refers to module.exports object
console.log(this);
const obj1 = {
data: "obj1 data",
met1: function () {
console.log(this.data);
},
met2: () => {
console.log(this.data);
},
};
const obj2 = {
data: "obj2 data",
test1: function () {
console.log(this.data);
},
test2: function () {
console.log(this.data);
}.bind(obj1),
test3: obj1.met1,
test4: obj1.met2,
};
obj2.test1();
obj2.test2();
obj2.test3();
obj2.test4();
obj1.met1.call(obj2);
Output:
Let me walk you through the outputs one by one (ignoring the first log starting from the second):
this is obj2 because of the left of the dot rule, we can see how test1 is called obj2.test1();. obj2 is left of the dot and thus the this value.
Even though obj2 is left of the dot, test2 is bound to obj1 via the bind() method. The this value is obj1.
obj2 is left of the dot from the function which is called: obj2.test3(). Therefore obj2 will be the value of this.
In this case: obj2.test4() obj2 is left of the dot. However, arrow functions don't have their own this binding. Therefore it will bind to the this value of the outer scope which is the module.exports an object which was logged in the beginning.
We can also specify the value of this by using the call function. Here we can pass in the desired this value as an argument, which is obj2 in this case.
Let's say I use a certain set of boilerplate fairly regularly:
class Foo {
method abc($a: $b, $c, +#d) is pure {
use Slang::Bar;
…
}
method xyz($a: $b, $c, +#d) is pure {
use Slang::Bar;
…
}
method blarg($a: $b, $c, +#d) is pure {
use Slang::Bar;
…
}
}
I'd rather be able to just say:
class Foo is/does Bar {
bar abc { … }
bar xyz { … }
bar blarg { … }
}
And somewhere in Bar, set up the declaration for bar (or, since class Foo will itself ultimately use its own declarator, it could go somewhere else and doesn't have to be pulled out in a separate Type). How would I go about doing that?
-1. Limitations (only for packages)
The method EXPORTHOW calls .set_how on current $?LANG adding a slang to the latter.
Then it add_package_declarator to the MAIN $?LANG which adds a package_declarator method to its Actions and Grammar. It is, I think, the only "dynamic slang" (in World.nqp).
If what you want is to overwrite routine_declarator. Then you have to write a slang imitating the chain just cited.
If you accept to keep the method keyword and make the automatic signature in the class, say according to the method name, here is a way:
Note: A Package is a container (package, grammar, module, role, knowhow, enum, class, subset). If you put code inside like a method, this gets executed (I'v just tried):
0. Description (EXPORTHOW)
I would use undocumented EXPORTHOW and DECLARE in a module because I did not find a way with Phaser. Apparently it is too late even at BEGIN.
The example I give, is decorating every method in a class (even BUILDALL).
1. Lib (decorator.rakumod)
class DecoratedClassHOW is Metamodel::ClassHOW {
method add_method(Mu $obj, $name, $code_obj) {
sub wrapper ($obj, $a, $b) {
say "Before $name";
my $res = $code_obj($obj, $a, $b);
say "After $name";
return $res;
}
my $res = callwith($obj, $name, &wrapper);
return $res;
}
}
my module EXPORTHOW {
package DECLARE {
constant decorated = DecoratedClassHOW;
}
}
2. Executable
use lib '.';
use decorator-lib;
decorated Foo {
method abc($a, $b) {
say "In abc: $a:$b";
}
}
my $f = Foo.new;
$f.abc(1, 2);
3. Output
Before BUILDALL
After BUILDALL
Before abc
In abc: 1:2
After abc
4. Sources
Grammar::Debugger: exporting a symbol (grammar) overriding find_method and add_method
And npq's NQPTraceHOW::trace-on: Looping with for $_.HOW.method_table($_) creating a new hash overwriting the method cache with the (well-named) nqp::setmethcache.
To automatize the signature, play with .signature
Jnthn article on EXPORTHOW
So post on EXPORTHOW impossible with role
I have a constructor function which registers an event handler:
function MyConstructor(data, transport) {
this.data = data;
transport.on('data', function () {
alert(this.data);
});
}
// Mock transport object
var transport = {
on: function(event, callback) {
setTimeout(callback, 1000);
}
};
// called as
var obj = new MyConstructor('foo', transport);
However, I'm not able to access the data property of the created object inside the callback. It looks like this does not refer to the object that was created, but to another one.
I also tried to use an object method instead of an anonymous function:
function MyConstructor(data, transport) {
this.data = data;
transport.on('data', this.alert);
}
MyConstructor.prototype.alert = function() {
alert(this.name);
};
but it exhibits the same problems.
How can I access the correct object?
What you should know about this
this (aka "the context") is a special keyword inside each function and its value only depends on how the function was called, not how/when/where it was defined. It is not affected by lexical scopes like other variables (except for arrow functions, see below). Here are some examples:
function foo() {
console.log(this);
}
// normal function call
foo(); // `this` will refer to `window`
// as object method
var obj = {bar: foo};
obj.bar(); // `this` will refer to `obj`
// as constructor function
new foo(); // `this` will refer to an object that inherits from `foo.prototype`
To learn more about this, have a look at the MDN documentation.
How to refer to the correct this
Use arrow functions
ECMAScript 6 introduced arrow functions, which can be thought of as lambda functions. They don't have their own this binding. Instead, this is looked up in scope just like a normal variable. That means you don't have to call .bind. That's not the only special behavior they have, please refer to the MDN documentation for more information.
function MyConstructor(data, transport) {
this.data = data;
transport.on('data', () => alert(this.data));
}
Don't use this
You actually don't want to access this in particular, but the object it refers to. That's why an easy solution is to simply create a new variable that also refers to that object. The variable can have any name, but common ones are self and that.
function MyConstructor(data, transport) {
this.data = data;
var self = this;
transport.on('data', function() {
alert(self.data);
});
}
Since self is a normal variable, it obeys lexical scope rules and is accessible inside the callback. This also has the advantage that you can access the this value of the callback itself.
Explicitly set this of the callback - part 1
It might look like you have no control over the value of this because its value is set automatically, but that is actually not the case.
Every function has the method .bind [docs], which returns a new function with this bound to a value. The function has exactly the same behavior as the one you called .bind on, only that this was set by you. No matter how or when that function is called, this will always refer to the passed value.
function MyConstructor(data, transport) {
this.data = data;
var boundFunction = (function() { // parenthesis are not necessary
alert(this.data); // but might improve readability
}).bind(this); // <- here we are calling `.bind()`
transport.on('data', boundFunction);
}
In this case, we are binding the callback's this to the value of MyConstructor's this.
Note: When a binding context for jQuery, use jQuery.proxy [docs] instead. The reason to do this is so that you don't need to store the reference to the function when unbinding an event callback. jQuery handles that internally.
Set this of the callback - part 2
Some functions/methods which accept callbacks also accept a value to which the callback's this should refer to. This is basically the same as binding it yourself, but the function/method does it for you. Array#map [docs] is such a method. Its signature is:
array.map(callback[, thisArg])
The first argument is the callback and the second argument is the value this should refer to. Here is a contrived example:
var arr = [1, 2, 3];
var obj = {multiplier: 42};
var new_arr = arr.map(function(v) {
return v * this.multiplier;
}, obj); // <- here we are passing `obj` as second argument
Note: Whether or not you can pass a value for this is usually mentioned in the documentation of that function/method. For example, jQuery's $.ajax method [docs] describes an option called context:
This object will be made the context of all Ajax-related callbacks.
Common problem: Using object methods as callbacks/event handlers
Another common manifestation of this problem is when an object method is used as callback/event handler. Functions are first-class citizens in JavaScript and the term "method" is just a colloquial term for a function that is a value of an object property. But that function doesn't have a specific link to its "containing" object.
Consider the following example:
function Foo() {
this.data = 42,
document.body.onclick = this.method;
}
Foo.prototype.method = function() {
console.log(this.data);
};
The function this.method is assigned as click event handler, but if the document.body is clicked, the value logged will be undefined, because inside the event handler, this refers to the document.body, not the instance of Foo.
As already mentioned at the beginning, what this refers to depends on how the function is called, not how it is defined.
If the code was like the following, it might be more obvious that the function doesn't have an implicit reference to the object:
function method() {
console.log(this.data);
}
function Foo() {
this.data = 42,
document.body.onclick = this.method;
}
Foo.prototype.method = method;
The solution is the same as mentioned above: If available, use .bind to explicitly bind this to a specific value
document.body.onclick = this.method.bind(this);
or explicitly call the function as a "method" of the object, by using an anonymous function as callback / event handler and assign the object (this) to another variable:
var self = this;
document.body.onclick = function() {
self.method();
};
or use an arrow function:
document.body.onclick = () => this.method();
Here are several ways to access the parent context inside a child context -
You can use the bind() function.
Store a reference to context/this inside another variable (see the below example).
Use ES6 Arrow functions.
Alter the code, function design, and architecture - for this you should have command over design patterns in JavaScript.
1. Use the bind() function
function MyConstructor(data, transport) {
this.data = data;
transport.on('data', ( function () {
alert(this.data);
}).bind(this) );
}
// Mock transport object
var transport = {
on: function(event, callback) {
setTimeout(callback, 1000);
}
};
// called as
var obj = new MyConstructor('foo', transport);
If you are using Underscore.js - http://underscorejs.org/#bind
transport.on('data', _.bind(function () {
alert(this.data);
}, this));
2. Store a reference to context/this inside another variable
function MyConstructor(data, transport) {
var self = this;
this.data = data;
transport.on('data', function() {
alert(self.data);
});
}
3. Arrow function
function MyConstructor(data, transport) {
this.data = data;
transport.on('data', () => {
alert(this.data);
});
}
It's all in the "magic" syntax of calling a method:
object.property();
When you get the property from the object and call it in one go, the object will be the context for the method. If you call the same method, but in separate steps, the context is the global scope (window) instead:
var f = object.property;
f();
When you get the reference of a method, it's no longer attached to the object. It's just a reference to a plain function. The same happens when you get the reference to use as a callback:
this.saveNextLevelData(this.setAll);
That's where you would bind the context to the function:
this.saveNextLevelData(this.setAll.bind(this));
If you are using jQuery you should use the $.proxy method instead, as bind is not supported in all browsers:
this.saveNextLevelData($.proxy(this.setAll, this));
You should know about "this" Keyword.
As per my view you can implement "this" in three ways
(Self|Arrow function|Bind Method)
A function's this keyword behaves a little differently in JavaScript compared to other languages.
It also has some differences between strict mode and non-strict mode.
In most cases, the value of this is determined by how a function is called.
It can't be set by assignment during execution, and it may be different each time the function is called.
ES5 introduced the bind() method to set the value of a function's this regardless of how it's called,
And ES2015 introduced arrow functions that don't provide their own this binding (it retains this value of the enclosing lexical context).
Method1: Self - Self is being used to maintain a reference to the original this even as the context is changing. It's a technique often used in event handlers (especially in closures).
Reference: this
function MyConstructor(data, transport) {
this.data = data;
var self = this;
transport.on('data', function () {
alert(self.data);
});
}
Method2: Arrow function - An arrow function expression is a syntactically compact alternative to a regular function expression, although without its own bindings to the this, arguments, super, or new.target keywords.
Arrow function expressions are ill-suited as methods, and they cannot be used as constructors.
Reference: Arrow function expressions
function MyConstructor(data, transport) {
this.data = data;
transport.on('data',()=> {
alert(this.data);
});
}
Method 3: Bind - The bind() method creates a new function that, when called, has its this keyword set to the provided value with a given sequence of arguments preceding any provided when the new function is called.
Reference: Function.prototype.bind()
function MyConstructor(data, transport) {
this.data = data;
transport.on('data',(function() {
alert(this.data);
}).bind(this);
The trouble with "context"
The term "context" is sometimes used to refer to the object referenced by this. Its use is inappropriate, because it doesn't fit either semantically or technically with ECMAScript's this.
"Context" means the circumstances surrounding something that adds meaning, or some preceding and following information that gives extra meaning. The term "context" is used in ECMAScript to refer to execution context, which is all the parameters, scope, and this within the scope of some executing code.
This is shown in ECMA-262 section 10.4.2:
Set the ThisBinding to the same value as the ThisBinding of the
calling execution context
Which clearly indicates that this is part of an execution context.
An execution context provides the surrounding information that adds meaning to the code that is being executed. It includes much more information than just the thisBinding.
The value of this isn't "context". It's just one part of an execution context. It's essentially a local variable that can be set by the call to any object and in strict mode, to any value at all.
First, you need to have a clear understanding of scope and behaviour of the this keyword in the context of scope.
this & scope:
There are two types of scope in JavaScript. They are:
Global Scope
Function Scope
In short, global scope refers to the window object. Variables declared in a global scope are accessible from anywhere.
On the other hand, function scope resides inside of a function. A variable declared inside a function cannot be accessed from the outside world normally.
The this keyword in the global scope refers to the window object. this inside a function also refers to the window object. So this will always refer to the window until we find a way to manipulate this to indicate a context of our own choosing.
--------------------------------------------------------------------------------
- -
- Global Scope -
- (globally "this" refers to window object) -
- -
- function outer_function(callback){ -
- -
- // Outer function scope -
- // Inside the outer function, the "this" keyword -
- // refers to window object -
- callback() // "this" inside callback also refers to the window object -
- } -
- -
- function callback_function(){ -
- -
- // Function to be passed as callback -
- -
- // Here "THIS" refers to the window object also -
- } -
- -
- outer_function(callback_function) -
- // Invoke with callback -
- -
--------------------------------------------------------------------------------
Different ways to manipulate this inside callback functions:
Here I have a constructor function called Person. It has a property called name and four method called sayNameVersion1, sayNameVersion2, sayNameVersion3, and sayNameVersion4. All four of them has one specific task. Accept a callback and invoke it. The callback has a specific task which is to log the name property of an instance of Person constructor function.
function Person(name){
this.name = name
this.sayNameVersion1 = function(callback){
callback.bind(this)()
}
this.sayNameVersion2 = function(callback){
callback()
}
this.sayNameVersion3 = function(callback){
callback.call(this)
}
this.sayNameVersion4 = function(callback){
callback.apply(this)
}
}
function niceCallback(){
// Function to be used as callback
var parentObject = this
console.log(parentObject)
}
Now let's create an instance from person constructor and invoke different versions of sayNameVersionX (X refers to 1,2,3,4) method with niceCallback to see how many ways we can manipulate the this inside callback to refer to the person instance.
var p1 = new Person('zami') // Create an instance of Person constructor
bind:
What bind do is to create a new function with the this keyword set to the provided value.
sayNameVersion1 and sayNameVersion2 use bind to manipulate this of the callback function.
this.sayNameVersion1 = function(callback){
callback.bind(this)()
}
this.sayNameVersion2 = function(callback){
callback()
}
The first one binds this with a callback inside the method itself. And for the second one, the callback is passed with the object bound to it.
p1.sayNameVersion1(niceCallback) // pass simply the callback and bind happens inside the sayNameVersion1 method
p1.sayNameVersion2(niceCallback.bind(p1)) // uses bind before passing callback
call:
The first argument of the call method is used as this inside the function that is invoked with call attached to it.
sayNameVersion3 uses call to manipulate the this to refer to the person object that we created, instead of the window object.
this.sayNameVersion3 = function(callback){
callback.call(this)
}
And it is called like the following:
p1.sayNameVersion3(niceCallback)
apply:
Similar to call, the first argument of apply refers to the object that will be indicated by the this keyword.
sayNameVersion4 uses apply to manipulate this to refer to a person object
this.sayNameVersion4 = function(callback){
callback.apply(this)
}
And it is called like the following. Simply the callback is passed,
p1.sayNameVersion4(niceCallback)
We can not bind this to setTimeout(), as it always executes with the global object (Window). If you want to access the this context in the callback function then by using bind() to the callback function, we can achieve it as:
setTimeout(function(){
this.methodName();
}.bind(this), 2000);
The question revolves around how the this keyword behaves in JavaScript. this behaves differently as below,
The value of this is usually determined by a function execution context.
In the global scope, this refers to the global object (the window object).
If strict mode is enabled for any function then the value of this will be undefined as in strict mode, global object refers to undefined in place of the window object.
The object that is standing before the dot is what the this keyword will be bound to.
We can set the value of this explicitly with call(), bind(), and apply()
When the new keyword is used (a constructor), this is bound to the new object being created.
Arrow functions don’t bind this — instead, this is bound lexically (i.e., based on the original context)
As most of the answers suggest, we can use the arrow function or bind() Method or Self var. I would quote a point about lambdas (arrow function) from Google JavaScript Style Guide
Prefer using arrow functions over f.bind(this), and especially over
goog.bind(f, this). Avoid writing const self = this. Arrow functions
are particularly useful for callbacks, which sometimes pass unexpectedly
additional arguments.
Google clearly recommends using lambdas rather than bind or const self = this
So the best solution would be to use lambdas as below,
function MyConstructor(data, transport) {
this.data = data;
transport.on('data', () => {
alert(this.data);
});
}
References:
https://medium.com/tech-tajawal/javascript-this-4-rules-7354abdb274c
arrow-functions-vs-bind
Currently there is another approach possible if classes are used in code.
With support of class fields, it's possible to make it the following way:
class someView {
onSomeInputKeyUp = (event) => {
console.log(this); // This refers to the correct value
// ....
someInitMethod() {
//...
someInput.addEventListener('input', this.onSomeInputKeyUp)
For sure under the hood it's all the old good arrow function that binds context, but in this form it looks much more clear that explicit binding.
Since it's a Stage 3 Proposal, you will need Babel and appropriate Babel plugin to process it as for now (08/2018).
I was facing a problem with Ngx line chart xAxisTickFormatting function which was called from HTML like this: [xAxisTickFormatting]="xFormat".
I was unable to access my component's variable from the function declared. This solution helped me to resolve the issue to find the correct this.
Instead of using the function like this:
xFormat (value): string {
return value.toString() + this.oneComponentVariable; //gives wrong result
}
Use this:
xFormat = (value) => {
// console.log(this);
// now you have access to your component variables
return value + this.oneComponentVariable
}
Another approach, which is the standard way since DOM2 to bind this within the event listener, that let you always remove the listener (among other benefits), is the handleEvent(evt) method from the EventListener interface:
var obj = {
handleEvent(e) {
// always true
console.log(this === obj);
}
};
document.body.addEventListener('click', obj);
Detailed information about using handleEvent can be found here: DOM handleEvent: a cross-platform standard since year 2000
Some other people have touched on how to use the .bind() method, but specifically here is how you can use it with .then() if anyone is having trouble getting them to work together:
someFunction()
.then(function(response) {
//'this' wasn't accessible here before but now it is
}.bind(this))
As mentioned in the comments, an alternative would be to use an arrow function that doesn't have its own 'this' value
someFunction()
.then((response)=>{
//'this' was always accessible here
})
this in JavaScript:
The value of this in JavaScript is 100% determined by how a function is called, and not how it is defined. We can relatively easily find the value of this by the 'left of the dot rule':
When the function is created using the function keyword the value of this is the object left of the dot of the function which is called
If there is no object left of the dot then the value of this inside a function is often the global object (global in Node.js and window in a browser). I wouldn't recommend using the this keyword here because it is less explicit than using something like window!
There exist certain constructs like arrow functions and functions created using the Function.prototype.bind() a function that can fix the value of this. These are exceptions of the rule, but they are really helpful to fix the value of this.
Example in Node.js
module.exports.data = 'module data';
// This outside a function in node refers to module.exports object
console.log(this);
const obj1 = {
data: "obj1 data",
met1: function () {
console.log(this.data);
},
met2: () => {
console.log(this.data);
},
};
const obj2 = {
data: "obj2 data",
test1: function () {
console.log(this.data);
},
test2: function () {
console.log(this.data);
}.bind(obj1),
test3: obj1.met1,
test4: obj1.met2,
};
obj2.test1();
obj2.test2();
obj2.test3();
obj2.test4();
obj1.met1.call(obj2);
Output:
Let me walk you through the outputs one by one (ignoring the first log starting from the second):
this is obj2 because of the left of the dot rule, we can see how test1 is called obj2.test1();. obj2 is left of the dot and thus the this value.
Even though obj2 is left of the dot, test2 is bound to obj1 via the bind() method. The this value is obj1.
obj2 is left of the dot from the function which is called: obj2.test3(). Therefore obj2 will be the value of this.
In this case: obj2.test4() obj2 is left of the dot. However, arrow functions don't have their own this binding. Therefore it will bind to the this value of the outer scope which is the module.exports an object which was logged in the beginning.
We can also specify the value of this by using the call function. Here we can pass in the desired this value as an argument, which is obj2 in this case.