JavaScript Array.prototype.find第二个参数thisArg不起作用

Question

I was reading a JavaScript book and found this code about how to use arr.find(callback[, thisArg])

class Person {
    constructor(name) {
        this.name = name;
        this.id = Person.nextId++;
    }
}

Person.nextId = 0;

const jamie = new Person("Jamie"),
      juliet = new Person("Juliet"),
      peter = new Person("Peter"),
      jay = new Person("Jay");
const arr = [jamie, juliet, peter, jay];

// option 2: using "this" arg:
arr.find(p => p.id === this.id, juliet); // returns juliet object

I cannot get the desired result. Everytime the find() returns undefined .

Answer 1

You are using arrow functions, which preserve lexical scoping. The this variable inside your arrow function represents window and not the juliet argument you have passed.

To rectify this, you can simply use function to create new scope and pass juliet as this .

 class Person { constructor(name) { this.name = name; this.id = Person.nextId++; } } Person.nextId = 0; const jamie = new Person("Jamie"), juliet = new Person("Juliet"), peter = new Person("Peter"), jay = new Person("Jay"); const arr = [jamie, juliet, peter, jay]; // option 2: using "this" arg: let a = arr.find(function(p) { return p.id === this.id; }, juliet); console.log(a);