30秒学会 JavaScript 片段 – Understanding the “this” keyword in JavaScript

What is this?

In JavaScript, the this keyword refers to the object that is currently executing the code. The short version of what this evaluates to is as follows:

• By default, this refers to the global object.
• In a function, when not in strict mode, this refers to the global object.
• In a function, when in strict mode, this is undefined.
• In an arrow function, this retains the value of the enclosing lexical context’s this.
• In an object method, this refers to the object the method was called on.
• In a constructor call, this is bound to the new object being constructed.
• In an event handler, this is bound to the element on which the listener is placed.

Global context

In the global execution context, this refers to the global object.

代码实现

console.log(this === window); // true

Function context

When not in strict mode, a function’s this refers to the global object.

使用样例

function f() {
  return this;
}

console.log(f() === window); // true

When in strict mode, a function’s this will be undefined if not set when entering the execution context.

'use strict';

function f() {
  return this;
}

console.log(f()); // undefined

Object context

When a function is called as a method of an object, this refers to the object the method is called on. This applies to methods defined anywhere in the object’s prototype chain (i.e. own and inherited methods).

const obj = {
  f: function() {
    return this;
  }
};

const myObj = Object.create(obj);
myObj.foo = 1;

console.log(myObj.f()); // { foo: 1 }

Similarly, when used inside a constructor, this refers to the object being constructed.

class C {
  constructor() {
    this.x = 10;
  }
}

const obj = new C();
console.log(obj.x); // 10

Arrow function context

In arrow functions, this retains the value of the enclosing lexical context’s this.

const f = () => this;

console.log(f() === window); // true

const obj = {
  foo: function() {
    const baz = () => this;
    return baz();
  },
  bar: () => this
};

console.log(obj.foo()); // { foo, bar }
console.log(obj.bar() === window); // true

Notice how in the second example, an arrow function’s this refers to the global object unless wrapped inside a regular function call, whose this refers to the object it’s called from and its lexical context is retained by the arrow function.

Event handler context

When used in an event handler, this refers to the element on which the listener is placed.

const el = document.getElementById('my-el');

el.addEventListener('click', function() {
  console.log(this === el); // true
});

Binding this

Using Function.prototype.bind() returns a new function from an existing one, where this is permanently bound to the first argument of bind().

function f() {
  return this.foo;
}

const x = f.bind({foo: 'hello'});
console.log(x()); // 'hello'

Similarly, using Function.prototype.call() or Function.prototype.apply() will bind the called function’s this to the first argument of either of these functions only for this call.

function f() {
  return this.foo;
}

console.log(f.call({foo: 'hi'})); // 'hi'

翻译自：https://www.30secondsofcode.org/js/s/this