Why do vue custom instructions use square brackets?

Vue.js is a popular JavaScript framework. In Vue.js, developers can extend the core functionality of Vue.js by writing custom instructions. When writing custom directives, a common question is: why are square brackets used to wrap the directive name?

First, let’s take a look at how directives are defined in Vue.js:

Vue.directive('my-directive', {
  bind: function () {},
  inserted: function () {},
  update: function () {},
  componentUpdated: function () {},
  unbind: function () {}
})

In this code, we can see that the Vue.directive method accepts two parameters. The first parameter is the name of the directive, and the second parameter is an object containing each hook function. So why is the instruction name wrapped in square brackets?

In fact, the answer to this question is not complicated. In Vue.js, directive names wrapped in square brackets are often called "dynamic directives". That is, when using dynamic directives, the directive name is dynamically calculated based on data on the component instance. For example:

In this example, the name of the v-bind directive is dynamically calculated and its value is the value of the attributeName variable. In this way, when the value of attributeName changes, the name of the directive will change accordingly.

Similarly, you can also use dynamic instructions to specify the instruction name in custom instructions. For example:

Vue.directive('[my-directive]', {
  bind: function () {},
  inserted: function () {},
  update: function () {},
  componentUpdated: function () {},
  unbind: function () {}
})

Different from using static instruction names, in this example, we use square brackets to wrap the instruction name. This method allows us to specify the name and behavior of the instruction more flexibly when writing custom instructions.

In addition to dynamic instruction names, square brackets can also be used to dynamically calculate the parameter values ​​of the instruction. For example:

In this example, the v-validation directive accepts a parameter rule, whose value is also dynamically calculated. In this way, we can get the parameter name through the $arg variable in the custom instruction and use it in the instruction hook function.

In short, in Vue.js, square brackets are used to support dynamic directive names and parameter values. This method allows us to be more flexible and adapt to more scenarios when writing custom instructions.

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