javascript - 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。后续功能

"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，或者我的假设是错误的？

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。

通过检查 a function's [[ThisMode]] slot 的可能值的简单英语描述，我们可以在 ES2015 规范中轻松地看到这一点。 ，可以是lexical、strict 或global:

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)，它会使函数的严格/非严格状态与确定 this 的目的无关>-设置行为。