I want to know what is the difference in VueJs between these two types of declaration :
data() {
return {
foo = 'bar'
}
}
and this :
created() {
this.foo = 'bar'
}
I know that I an access both using 'this' in or in methods.
Also, if the returned object in data() is saved in "memory" of the component, where does the object declared in created() is saved? are they in different scopes?
Also, I know that to fetch data, the fetcher lands in created() and then updates the data object, but this question is specifically about the differences between the two ways of declarations i mentioned
Is there any differences in the background?
You can read more about vue data here.
Vue will recursively convert its ($data) properties into getter/setters to make it “reactive”..
created() {
this.foo = 'bar'
}
Declaring like above, you won't be able to watch the attributes.
You can check this example out, watch function isn't fired with the attribute being not defined in data()
Related
Is there a way in Vue2 to do this:
....,
computed {
...mapState('myModule', {
myVal: (state, vm) => state.someVar.filter((_) => { return vm.someVar })
})
},
....
The actual function I'm working on is filtering the state.someVar in a more complex scenerio, so this is simplified. The point being that this is undefined.
Anyone know how to reference the vue in such a function?
The 2nd argument to the mapState function is supposed to be a state mapper (array or object)
If you are using an object (as you are), you can only set the properties of the object to the keys in your "myModule" state object. You cannot set them as functions
e.g {customName: moduleStateKey}
As for passing component instance, you can only do that in an action or mutation. However, that would make your store impure. It would be better to pass someVar to state via a mutation fired from the component, then use a getter for the filter logic and use the someVar passed to state there
I'm creating a new Audio() object inside a Vue component's data() { ... } function, but I got concerned about if this is unnecessarily creating more Audio objects.
I can't seem to find when or how the data() function is called, but I also couldn't find any examples that crates a new object in the data function.
Is it ok to create new objects inside data() ? Or should I leave those initializations to the created() function?
Read the documentation about data() : https://v2.vuejs.org/v2/guide/components.html#data-Must-Be-a-Function
a component’s data option must be a function, so that each instance can maintain an independent copy of the returned data object.
It is the right way to do it.
Your new object will be instantiated like so :
data() {
// new object returned
return {
audio: new Audio()
}
}
If you want to have a global object to avoid multiple instance of audio, think about attaching it to the global Vue instance or using Vuex depending on what you need.
It's completely fine to create new objects inside the returned object of the data() function, that's what is meant for. More informations here.
There are couple of questions related computed properties like the following
"vuejs form computed property"
"Computed properties in VueJs"
"computed property in VueJS"
"Use computed property in data in Vuejs"
They are asking about specific error or logic. There are lot of websites that are explaining about vuejs related concepts. I read about computed properties on vuejs official website. When we do complex calculations or want to avoid to write more logic in our html template then we use computed properties.
But could not get any solid understanding about computed properties, when it calls, how it calls, what exactly do?
TL;DR: Computed properties are getters/setters in Vue.
When defined in the shorthand form, they are getters:
computed: {
someComputed() {
return `${this.foo} ${this.bar}`;
}
}
is equivalent with
computed: {
someComputed: {
get: function() {
return `${this.foo} ${this.bar}`;
}
}
}
which can also have a setter:
computed: {
someComputed: {
get: function() {
return `${this.foo} ${this.bar}`;
}
set: function(fooBar) {
const fooBarArr = fooBar.split(' ');
this.foo = fooBarArr[0];
this.bar = fooBarArr[1];
}
}
}
In short, Vue computed properties allow you to bind an instance property to
a getter: function run when you look up that property; usage:
this.someComputed // returns the computed current value, running the getter.
a setter: function run when you attempt to assign that property; usage:
this.someComputed = value; // sets the computed current value, running the setter.
Read more on getters and setters in Javascript.
And here's the documentation on Vue computed properties.
You can use computed properties when for example you have some logic what will blow up your template.
The idea is, that normally you want to keep all javascript logic in the javascript side of your vue component, and only access final data in your data (if possible)
For that you can use computed props, which normally are doing simple things like:
computed: {
// a computed getter
reversedMessage: function () {
// `this` points to the vm instance
return this.message.split('').reverse().join('')
}
}
Or an another good example if you have some currency and you want to format it with thousand separator and euro ign at the end.
Then you can access your computed prop in the template like you access a normal prop, you dont have to call it as a function.
like so:
<div>{{reversedMesage}}</div>
Every time, when any variable what is used in your conputed prop is changing, vue vill take care of it and will re-calculate your computed property again.
Lets say you have the following:
computed: {
prettyAmount: function () {
return this.amount + ' ' + this.currency.toUpperCase()
}
}
<div>{{prettyAmount}}</div>
Whenever currency or amount changes, the output of prettyAmount will be changed as well.
I have a Vue(2.5+) component where I'm setting a data property to a new Foo object. Using foo.bar() in the click handler calls the method correctly, but throws Uncaught TypeError: cannot set property 'someVariable' of null when trying to modify properties inside the Foo class. Setting it up so that Foo is an object literal instead of a class also does not resolve the error.
I suspect something weird is happening with this, between the component and the class?
Vue component
import Foo from './foo.js'
export default {
template: `<div #click="foo.bar"></div>`,
data() {
return {
foo: new Foo()
}
},
created() {
console.log(foo); // foo is not null here
}
}
Foo class
export default class Foo
{
constructor()
{
this.someVariable = 0;
}
bar(e)
{
// modify this.someVariable
}
}
but if I change the vue component to reference the external method through it's own "methods" property, it works.
Vue component (working)
import Foo from './foo.js'
export default {
template: `<div #click="bar"></div>`,
data() {
return {
foo: new Foo()
}
},
methods: {
bar(e) {
this.foo.bar(e);
}
}
}
As said in the comments, foo.bar without any context attached to it :
In JS functions are objects, just like any object they have their own this "pointer".
In the evaluation of their body, this is bound to a specific object referred to as context which is either the default context (automatically set) or user defined (manually set).
Inheritance in JS is achieved through a prototype chain and methods should be defined on/attached to the class's prototype. Because of this, when you call foo.bar() :
You are in a method call context, therefore foo will be bound to the method
bar is searched on the object first then in the prototype chain
But methods behave just like any other property : when you do foo.bar you get a reference to the actual method which is an unbound function (default behavior for methods, since it is bound when called on an object).
Therefore, what you really need to do in this situation is foo.bar.bind(foo).
I would also suggest taking a quick look into this ES6 proposal for a bind operator and its implementation as a Babel plugin which allows nice things like passing ::foo.bar instead of foo.bar.bind(foo)
I have a component which has two functions in it. One function returns an array of objects while the other receives it as a parameter. Both the functions being in the same component.
Example Code:
export default class Test extends Component {
func1 () {
return arrayofobj
}
param = this.func1();
func2 (param) {
param.id
}
}
So, my question is. How can we pass and access the array of objects to "func2" i.e. param.id
Also, I do not want to use state or props in this case.
You do not really understand what a class and component are. Try to learn about it here: https://facebook.github.io/react/docs/react-component.html
https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Classes