call、apply和bind方法

1. call和apply

call和apply方法非常相似，都是先设置函数中的this操作符的值，并且执行这个函数。

区别：

1. apply以数组形式接受传递给函数的参数
2. call以单个分开参数的形式

例子：

class Person {
    public name: string;
    public surname: string;
    
    constructor(name: string, surname: string) {
        this.name = name;
        this.surname = surname;
    }
    
    public greet(city: string, country: string) {
        // 使用this操作符访问实例的name和surname属性
        var msg = `Hi, my name is ${this.name} ${this.surname}. `;
        msg += `I'm from ${city} (${country}).`;
        console.log(msg);
    }
}


var person = new Person("Fourteen", "Zhang");

person.greet("Shijiazhuang", "China");
// Hi, my name is Fourteen Zhang. I'm from Shijiazhuang (China).

// 通过call和apply函数调用该方法
person.greet.call(person, "Shijiazhuang", "China");
person.greet.apply(person, ["Shijiazhuang", "China"]);

var valueOfThis = { name: "Fourteen", surname: "Zhang" };
person.greet.call(valueOfThis, "Shijiazhuang", "China");
person.greet.apply(valueOfThis, ["Shijiazhuang", "China"]);
// Hi, my name is Fourteen Zhang. I'm from Shijiazhuang (China).

2. bind

bind方法也可以设置this操作符的值，但不执行它。

注意：

1. 调用函数的bind方法时，返回一个和原函数具有相同函数体和作用域的新函数
2. 函数体内部的this操作符指向的值，已被永久地改变为传递给bind方法的第一个参数，不会改变

例子：

var person = new Person("Fourteen1", "Zhang");

var greet = person.greet.bind(person);

greet.call(person, "Hebei", "China");
greet.apply(person, ["Hebei", "China"]);
// Hi, my name is Fourteen1 Zhang. I'm from Shijiazhuang (China).

greet.call(null, "Hebei", "China");
greet.apply(null, ["Hebei", "China"]);
// Hi, my name is Fourteen1 Zhang. I'm from Hebei (China).

var valueOfThis = { name: "Fourteen2", surname: "Zhang" };
greet.call(valueOfThis, "Hebei", "China");
greet.apply(valueOfThis, ["Hebei", "China"]);
// Hi, my name is Fourteen1 Zhang. I'm from Hebei (China).

一旦使用bind方法为一个函数内的this操作符进行了绑定，就不能再覆盖它：

var valueOfThis = { name: "Fourteen3", surname: "Zhang" };

var greet = person.greet.bind(valueOfThis);
greet.call(valueOfThis, "Hebei", "China");
greet.apply(valueOfThis, ["Hebei", "China"]);
// Hi, my name is Fourteen1 Zhang. I'm from Hebei (China).