Call and apply in Javascript

In js, call and apply are used to bind functions to another object for running. The two functions are different only when defining parameters, next I will introduce the call and apply usage.

In the web Front-end development process, we often need to change this point. Generally, we want to use the call method, but many people do not have a clear understanding of the call, the following small series will give you a detailed description of call and apply.

1. Definition of the call Method

You can search for call in Baidu. The definition of call is very easy. In my understanding, a. call (B, arg1, arg2.) is the method of object a applied to object B. For example:

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Function add (a, B) { Alert (a + B ); } Function reduce (a, B) { Alert (a-B ); } Add. call (reduce, 1, 3) // apply the add method to reduce. The result is 4.

2. call can change this point

For example:

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Function B () { Alert (this) } B (); // window B. call (); // window B. call ("a", 2, 3); //

Let's look at a complicated example:

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Function Animal () { This. name = "animal "; This. showName = function () { Alert (this. name) } } Function Cat () { This. name = "cat "; } Var animal = new Animal (); Var cat = new Cat (); Animal. showName (); // The result is animal. Animal. showName. call (cat); // The original cat has no showName method, but the showName method of animal is applied to cat through the call method. Therefore, the result is cat

3. Implement inheritance

Example:

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Function Animal (name) { This. name = name; This. showName = function () { Alert (this. name) } } Function Cat (name) { Animal. call (this, name); // apply Animal to Cat, so Cat has all attributes and methods of Animal. } Var cat = new Cat ("Black Cat "); Cat. showName (); // Black Cat is displayed in the browser.

Iv. apply usage

The usage of apply and call is different from that of call.

A. call (B, arg1, arg2 ...)

Apply (B, [arg1, arg2]) // apply has only two parameters, which call the parameters (arg1, arg2 ...) Put it in an array as the second parameter of apply