How to use this in js

What does this in js mean?

This is always used in JS. Today I will talk to you about what this is that can easily confuse people and how to use this.

This points to 1, ordinary function points to the caller of the function: a simple way is to see if there is a dot in front of the function. If there is a dot, then point to the value in front of the dot; 2. The arrow function points to the domain where the function is located: Pay attention to understanding the scope. Only the {} of the function constitutes the scope. The {} of the object and if(){} do not constitute the scope;

const obj = {
name: 'objName',
say() {
console.log(this.name);  
      }, 
read: () => {
     console.log(this.name);
             }
       }
obj.say(); // objName
obj.read(); // undefined

Ordinary function, the caller is obj, so the result is objname; also understand that say() is an ordinary function, with a dot in front, so this points to obj;

arrow Function, this points to the scope where the function is located. The current scope is the global environment, so this.name is undefined.

Take the following example to understand more clearly where the arrow function this points, where the arrow function is located The scope is the ordinary function say, and the caller of say() is obj

const obj = { 
say: function () {
  setTimeout(() => {
   console.log(this)
                  });
                 }
             }

obj.say(); // obj, at this time this refers to the definition of him obj

function test() {  
console.log(this);
}

is {}, and the default this for ordinary functions is global

console.log(this); // {}
function test() {
 console.log(this);
}
test(); //global

Come on two questions to check whether you have mastered it

example1

const length = 10;
function fn() {
console.log(this.length);
}
const  obj = {
  length: 5,
  method: function(fn) { 
  fn();
  arguments[0]();   
  }
};
obj.method(fn, 1);

Output 10, 2

I just saw this question at the beginning I was also confused about the question, and now I finally understand it. The method function passes in two parameters, one parameter is fn(), fn() is an ordinary function, and this points to the caller of the function, which at this time points to the whole world (you can also See that there is no dot in front of this function), so the running result is 10. arguments are all parameters of the function, which is an object of class

, arguments[0] (), which can be regarded as arguments.0() , what calls this function is arguments, at this time this refers to arguments, this.length refers to angument.length, which is the total number of parameters passed in 2

Note: The running result of the above example in the node environment is undefined 2, const length = 10 is changed to global.length = 10; because the

defined globally in the node environment will not be bound to global, and the browser It will also be automatically bound to the global environment windowWhat will be the result if it is changed to the following?

const length = 10;
function fn() {
    console.log(this.length);
}
const obj = {
    length: 5,
    method: function(fn) {
        fn();
        const fun = arguments[0];
        fun()；
    }
};
obj.method(fn, 1);
10, 10
example 2
window.val = 1;
var obj = {
    val: 2,
    dbl: function() {
        this.val *= 2
        val *= 2;
        console.log(val);
        console.log(this.val);
    }
}
obj.dbl(); // 2 4
var func = obj.dbl;
func(); // 8 8

This is a bit roundabout, but step by step It is easy to understand when analyzing:

obj.dbl(); When executing this line of code, this refers to obj, so this.val === obj.val*=2, and the final result is 4 ,val*=2 === window.val *= 2, the final result is 2

func(), when executing this line of code, func() does not have any prefix, this refers to window.func( ); So at this time the value of this is window, this.val === window.val *= 2, at this time window.val is 4, val*=2 === window.val *2, the final result is 8, finally console.log(this.val), and console.log(val) both refer to window.val.

The above is my little understanding of this. If you have any questions about this article of mine, please feel free to communicate with me.

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