Detailed explanation of usage examples of this in JS
This time I will bring you a detailed explanation of the use of this pointer in JS. What are the precautions when using this pointer in JS. The following is a practical case, let's take a look.
In specific practical applications, the point of this cannot be determined when the function is defined, but is determined when the function is executed. It can be roughly divided into the following three types according to the execution environment:
1. When the function is called as a normal function, this points to the global object
2. When the function is called as a method of the object, this points to the object
3. When the function is used as a constructor When the caller is called, this points to the newly created object
Example 1:
window.name = 'myname';
function getName() {
console.log(this.name);
}
getName(); //输出mynameExample 2:
var boy = {
name: 'Bob',
getName: function() {
console.log(this.name);
}
}
boy.getName(); //输出BobExample 3:
function Boy(name) {
this.name = name;
}
var boy1 = new Boy('Bob');
console.log(boy1.name); //输出BobFor Example 3, there is another special case, that is, when the constructor returns an object through "return", the value of this operation The final result is this object, not a newly created object, so this is of no use in this case.
Example four:
function Boy(name) {
this.name = name;
return { //返回一个对象
name: 'Jack'
}
}
var boy1 = new Boy('Bob');
console.log(boy1.name); //输出JackExample five:
function Boy(name) {
this.name = name;
return 1; //返回非对象
}
var boy1 = new Boy('Bob');
console.log(boy1.name); //输出BobThe role of call and apply
apply accepts two parameters. The first parameter specifies the pointer of this in the function body. The second parameter is an array or array-like used to pass the number of the called function. parameter list.
Example 1:
function getInfo() {
console.log(this.name+' like '+arguments[0]+' and '+arguments[1]);
}
var boy1 = {
name: 'Bob',
age: 12
}
getInfo.apply(boy1,['sing','swimming']); //输出Bob like sing and swimmingExample 2:
function getInfo() {
console.log(this.name+' like '+arguments[0]+' and '+arguments[1]);
}
var boy1 = {
name: 'Bob',
age: 12
}
getInfo.call(boy1,'sing','shopping'); //输出Bob like sing and shoppingExample 3: Simulate the browser's bind method
Function.prototype.bind = function(obj){
var self = this;
return function(){
return self.apply(obj,arguments);
}
};
var obj = {
name: 'Bob',
age: 12
};
var func = function(){
console.log(this.name+' like '+arguments[0]+' and '+arguments[1]);
}.bind(obj);
func('sing','shopping');The missing this
is in In some cases, the pointing of this will be lost. In this case, we need to use call, apply and bind to change the pointing of this. Example 1: When the "getName" method is called as a property of the "boy" object, this points to the "boy" object. When another variablereferences the "getName" method, because it It is called as an ordinary function, so this points to the global object window
var boy = {
name: 'Bob',
getName: function() {
console.log(this.name);
}
}
boy.getName(); //输出Bob
var getBoyName = boy.getName;
getBoyName(); //输出undefinedvar boy1 = {
name: 'Bob',
age: 12,
getInfo: function() {
console.log(this.name);
function getAge() {
console.log(this.age);
}
getAge();
}
}
boy1.getInfo(); //Bob
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