Knowledge and application of JavaScript function calls

JavaScript function calls play an important role in js. This article will provide some detailed understanding of them.

There are 4 ways to call JavaScript functions.

The difference between each method lies in the initialization of this.

this keyword

Generally speaking, in Javascript, this points to the current object when the function is executed.

#Note that this is a reserved keyword, you cannot modify the value of this.

Calling JavaScript functions

In the previous chapters we have learned how to create functions.

The code in the function is executed after the function is called.

Called as a function

Instance

function myFunction(a, b) {
    return a * b;}myFunction(10, 2);           // myFunction(10, 2) 返回 20

The above function does not belong to any object. But in JavaScript it is always the default global object.

The default global object in HTML is the HTML page itself, so the function belongs to the HTML page.

The page object in the browser is the browser window (window object). The above functions will automatically become functions of the window object.

myFunction() and window.myFunction() are the same:

Instance

function myFunction(a, b) {
    return a * b;}window.myFunction(10, 2);    // window.myFunction(10, 2) 返回 20

This is the call to the JavaScript function Commonly used methods, but not good programming habits
Global variables, methods or functions can easily cause naming conflict bugs.

Global object

When the function is not called by its own object, the value of this will become the global object.

In a web browser, the global object is the browser window (window object).

The value of this returned by this instance is the window object:

Instance

function myFunction() {
    return this;}myFunction();                // 返回 window 对象

The function is called as a global object, which will cause The value of this becomes the global object.
Using the window object as a variable can easily cause the program to crash.

Function as method call

In JavaScript you can define functions as methods of objects.

The following example creates an object (myObject), which has two properties (firstName and lastName), and one method (fullName):

Instance

var myObject = {
    firstName:"John",    lastName: "Doe",    fullName: function () {
        return this.firstName + " " + this.lastName;    }}myObject.fullName();         // 返回 "John Doe"

fullName method is a function. Functions belong to objects. myObject is the owner of the function.

This object holds JavaScript code. The value of this in the instance is the myObject object.

Test the following! Modify the fullName method and return this value:

Instance

var myObject = {
    firstName:"John",    lastName: "Doe",    fullName: function () {
        return this;    }}myObject.fullName();          // 返回 [object Object] (所有者对象)

The function is called as an object method, which will make the value of this become the object itself.

Use the constructor to call the function

If the new keyword is used before the function call, the constructor is called.

This looks like a new function is created, but in fact JavaScript functions are recreated objects:

Instance

// 构造函数:function myFunction(arg1, arg2) {
    this.firstName = arg1;    this.lastName  = arg2;}
 // This    creates a new objectvar x = new myFunction("John","Doe");x.firstName;                             // 返回 "John"

The call to the constructor creates a new Object. The new object inherits the constructor's properties and methods.

#The this keyword in the constructor has no value.
The value of this is created when the function calls the instantiated object (new object).

Calling a function as a function method

In JavaScript, functions are objects. A JavaScript function has its properties and methods.

call() and apply() are predefined function methods. Two methods can be used to call functions, and the first parameter of both methods must be the object itself.

Example

function myFunction(a, b) {
    return a * b;}myObject = myFunction.call(myObject, 10, 2);     // 返回 20

Example

function myFunction(a, b) {
    return a * b;}myArray = [10, 2];myObject = myFunction.apply(myObject, myArray);  // 返回 20

Both methods use the object itself as the first parameter. The difference between the two lies in the second parameter: apply passes in a parameter array, that is, multiple parameters are combined into an array and passed in, while call is passed in as the parameter of call (starting from the second parameter).

In JavaScript strict mode (strict mode), the first parameter will become the value of this when calling the function, even if the parameter is not an object.

In JavaScript non-strict mode (non-strict mode), if the value of the first parameter is null or undefined, it will use the global object instead.

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