javascript中对象的this只自己本身，看如下代码
let user = { name: "John", age: 30, sayHi() { // "this" is the "current object" alert(this.name); } }; user.sayHi(); // John
这里的this只user对象，this.name翻译成user.name所以结果是John
那为什么不直接用user.name，因为如果直接使用外部变量有可能会报错，如下：
let user = { name: "John", age: 30, sayHi() { alert( user.name ); // leads to an error } }; let admin = user; user = null; // overwrite to make things obvious admin.sayHi(); // TypeError: Cannot read property 'name' of null
将外部变量赋予空值后，导致了程序报错
this是在程序运行时才进行绑定
let user = { name: "John" }; let admin = { name: "Admin" }; function sayHi() { alert( this.name ); } // use the same function in two objects user.f = sayHi; admin.f = sayHi; // these calls have different this // "this" inside the function is the object "before the dot" user.f(); // John (this == user) admin.f(); // Admin (this == admin) admin['f'](); // Admin (dot or square brackets access the method – doesn't matter)
箭头函数没有this
let user = { firstName: "Ilya", sayHi() { let arrow = () => alert(this.firstName); arrow(); } }; user.sayHi(); // Ilya
