call 和 apply，调用一个对象的函数，用另一个对象取代当前对象，this 指向转变。两者区别:参数个数。
function Person(fn, ln) {
this.first_name = fn;
this.last_name = ln;
this.displayName = function() {
console.log(`Name: ${this.first_name} ${this.last_name}`);
}
}
let person = new Person("John", "Reed");
person.displayName(); // Prints Name: John Reed
let person2 = new Person("Paul", "Adams");
person2.displayName(); // Prints Name: Paul Adams
person.displayName.call(person2); // Here we are setting value of this to be person2 object
//Prints Name: Paul Adams
bind：创建一个方法，并且 this 指针指向当前的指定的对象
function Person(fn, ln) {
this.first_name = fn;
this.last_name = ln;
this.displayName = function() {
console.log(`Name: ${this.first_name} ${this.last_name}`);
}
}
let person = new Person("John", "Reed");
person.displayName(); // Prints Name: John Reed
let person2 = new Person("Paul", "Adams");
person2.displayName(); // Prints Name: Paul Adams
let person2Display = person.displayName.bind(person2); // Creates new function with value of “this” equals to person2 object
person2Display(); // Prints Name: Paul Adams
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