在 javascript 中使用严格不适用于粗箭头？

Question

我发现了一个有趣的案例，其中 "use strict" 在 javascript 中没有按预期工作。以下函数

"use strict";

var y = () => {
    console.log(this);
}

var x = function () {
    console.log(this);
}

x(); // undefined due to use strict
y(); // window object

我觉得fat arrow context也应该被undefined覆盖，还是我的假设有误？

Answer 1

MDN 说 arrow functions:

Relation with strict mode

Given that this is lexical, strict mode rules with regard to this are just ignored.
var f = () => {'use strict'; return this};
f() === window; // or the global object

词法 this 规则优先于严格模式 this 规则。

我们可以在 ES2015 规范中通过检查 a function's [[ThisMode]] slot 可能值的简单英语描述轻松地看到这一点，这些值可以是 lexical、strict 或 [=18] =]:

Defines how this references are interpreted within the formal parameters and code body of the function. lexical means that this refers to the this value of a lexically enclosing function. strict means that the this value is used exactly as provided by an invocation of the function. global means that a this value of undefined is interpreted as a reference to the global object.

换句话说，函数的 this 行为可以是严格的、非严格的或词法的。如果函数的 [[ThisMode]] 是词法的（就像箭头函数一样），它会使函数的 strict/non-strict 状态与确定 this 设置行为无关。

在 javascript 中使用严格不适用于粗箭头？

use strict in javascript not working for fat arrow?

javascript

use-strict

ecmascript-6