array.forEach 的 thisArg 未按预期引用

Question

给定以下代码：

const theArray = ['Audi','Volvo','Mercedes'];

const myObj = {a: 7};

theArray.forEach((value, index, array) => {
    console.log(index + ' : ' + value);
    console.log(array === theArray);
    console.log(this.a);
}, myObj);

我得到以下输出：

0 : Audi
true
undefined
1 : Volvo
true
undefined
2 : Mercedes
true
undefined

我不明白为什么 this 不引用 myObj 和 returns undefined 而不是 7。虽然 this typeof Object returns 是真的，但我不知道它引用了哪个对象。我只知道 this returns 一个空对象（即 {}）

Node.js解释器版本为v6.2.1

V8-引擎版本为5.0.71.52

Answer 1

问题

Arrow functions:

An arrow function expression has a shorter syntax compared to function expressions and lexically binds the this value (does not bind its own this, arguments, super, or new.target). Arrow functions are always anonymous.

解决方案 1

使用function

const theArray = ['Audi','Volvo','Mercedes'];

const myObj = {a: 7};

theArray.forEach(function (value, index, array) {
    console.log(index + ' : ' + value);
    console.log(array === theArray);
    console.log(this.a);
}, myObj);

解决方案 2

使用闭包

var abc = 'abc';
const theArray = ['Audi','Volvo','Mercedes'];

const myObj = {a: 7};

theArray.forEach((obj => (value, index, array) => {
    console.log(index + ' : ' + value);
    console.log(array === theArray);
    console.log(obj.a);
    console.log(this.abc);
})(myObj));

array.forEach 的 thisArg 未按预期引用

thisArg of array.forEach does not reference as expected

javascript

foreach

object-reference

node.js