Vue.js Basics


Example

In addition to the default set of directives shipped in core, Vue.js also allows you to register custom directives. Custom directives provide a mechanism for mapping data changes to arbitrary DOM behavior.

You can register a global custom directive with the Vue.directive(id, definition) method, passing in a directive id followed by a definition object. You can also register a local custom directive by including it in a component’s directives option.

Hook Functions

  • bind: called only once, when the directive is first bound to the element.
  • update: called for the first time immediately after bind with the initial value, then again whenever the binding value changes. The new value and the previous value are provided as the argument.
  • unbind: called only once, when the directive is unbound from the element.
Vue.directive('my-directive', {
     bind: function () {
       // do preparation work
       // e.g. add event listeners or expensive stuff
       // that needs to be run only once
     },
     update: function (newValue, oldValue) {
       // do something based on the updated value
       // this will also be called for the initial value
     },
     unbind: function () {
       // do clean up work
       // e.g. remove event listeners added in bind()
     }    
})

Once registered, you can use it in Vue.js templates like this (remember to add the v- prefix):

<div v-my-directive="someValue"></div>

When you only need the update function, you can pass in a single function instead of the definition object:

Vue.directive('my-directive', function (value) {
  // this function will be used as update()
})

Directive Instance Properties

All the hook functions will be copied into the actual directive object, which you can access inside these functions as their this context. The directive object exposes some useful properties:

  • el: the element the directive is bound to.
  • vm: the context ViewModel that owns this directive.
  • expression: the expression of the binding, excluding arguments and filters.
  • arg: the argument, if present.
  • name: the name of the directive, without the prefix.
  • modifiers: an object containing modifiers, if any.
  • descriptor: an object that contains the parsing result of the entire directive.
  • params: an object containing param attributes. Explained below.

You should treat all these properties as read-only and never modify them. You can attach custom properties to the directive object too, but be careful not to accidentally overwrite existing internal ones.

An example of a custom directive using some of these properties:

HTML

<div id="demo" v-demo:hello.a.b="msg"></div>

JavaScript

Vue.directive('demo', {
  bind: function () {
    console.log('demo bound!')
  },
  update: function (value) {
    this.el.innerHTML =
      'name - '       + this.name + '<br>' +
      'expression - ' + this.expression + '<br>' +
      'argument - '   + this.arg + '<br>' +
      'modifiers - '  + JSON.stringify(this.modifiers) + '<br>' +
      'value - '      + value
  }
})
var demo = new Vue({
  el: '#demo',
  data: {
    msg: 'hello!'
  }
})

Result

name - demo
expression - msg
argument - hello
modifiers - {"b":true,"a":true}
value - hello!

Object Literal

If your directive needs multiple values, you can also pass in a JavaScript object literal. Remember, directives can take any valid JavaScript expression:

HTML

<div v-demo="{ color: 'white', text: 'hello!' }"></div>

JavaScript

Vue.directive('demo', function (value) {
  console.log(value.color) // "white"
  console.log(value.text) // "hello!"
})

Literal Modifier

When a directive is used with the literal modifier, its attribute value will be interpreted as a plain string and passed directly into the update method. The update method will also be called only once, because a plain string cannot be reactive.

HTML

<div v-demo.literal="foo bar baz">

JavaScript

Vue.directive('demo', function (value) {
  console.log(value) // "foo bar baz"
})