If I bind a class:
:class="['string-class', {'test-case-class': true}]
It will add both of the classes above, but how can I evaluate the above to a string form. I need to do this as a third party component I am using only accepts strings for a class property.
Edit
I want the above to be able to output: 'string-class test-case-class' just as you would see on a component that :class would be on.
You can bind the class to a function, rather than literals. For instance:
:class="getClasses"
where the function is computed:
computed: {
getClasses: function() {
return 'string-class test-case-class'
},
You can then use it just like any other computed function:
{{ getClasses }}
Should show the string literal 'string-class test-case-class'.
Related
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 ($data) properties into getter/setters to make it “reactive”..
created() {
this.foo = 'bar'
}
Declaring like above, you won't be able to watch the attributes.
You can check this example out, watch function isn't fired with the attribute being not defined in data()
There are couple of questions related computed properties like the following
"vuejs form computed property"
"Computed properties in VueJs"
"computed property in VueJS"
"Use computed property in data in Vuejs"
They are asking about specific error or logic. There are lot of websites that are explaining about vuejs related concepts. I read about computed properties on vuejs official website. When we do complex calculations or want to avoid to write more logic in our html template then we use computed properties.
But could not get any solid understanding about computed properties, when it calls, how it calls, what exactly do?
TL;DR: Computed properties are getters/setters in Vue.
When defined in the shorthand form, they are getters:
computed: {
someComputed() {
return `${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.
The problem is that I can't pass dynamic Pug variables to Vue component via attributes, if they're of "String" type. Vue considers the string that I'm trying to pass as a name of Vue property.
The problem
template.pug
- var pugVariable = 'John';
my-component(v-bind:name= pugVariable)
*MyComponent.Vue*
export default {
name: 'MyComponent',
props: {
name: {
type: String
}
}
}
I get an error: "Property or method "John" is not defined on the instance but referenced during render.", which means, as far as I understand, that Vue considers string that is in pugVariable as a name of Vue property.
The question
So, the question is: now to persuade Vue to treat this variable as a string?
What I've already tried:
I tried to pass an object literal instead of string, as follows:
my-component(v-bind:name= {value: pugVariable})
It works, but we lose the ability to check the type of passed value, so I don't like this solution.
I've got an answer earlier that whoever:)
As described here https://stackoverflow.com/a/45175556/7473709, in case if we want just to pass static string, we have to simply get rid of v-bind: prefix, so it looks like as follows:
my-component(name= {value: pugVariable})
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)
in vue guide doc, I have looked at the documentation about prop validation :
The type can be one of the following native constructors:
String
Number
Boolean
Function
Object
Array
In addition, type can also be a custom constructor function and the assertion will be made with an instanceof check.
so, how can I use the instanceof operator to achieve parameter custom type checking ?
you can use custom type like:
props: {
file: { type: File, required: true }
}