为什么 babel 将导入的函数调用重写为 (0, fn)(...)?

问题描述

给定一个输入文件

import { a } from 'b';

function x () {
  a()
}

babel 会编译成

'use strict';

var _b = require('b');

function x() {
  (0, _b.a)();
}

但是在松散模式下编译时，函数调用输出为 _b.a();

but when compiled in loose mode the function call is output as _b.a();

我已经对添加逗号运算符的位置进行了一些研究，希望有评论对其进行解释.负责添加它的代码是这里.

I've done some research into where the comma operator is added in the hope there was a comment explaining it.The code responsible for adding it is here.

推荐答案

(0, _b.a)() 确保函数 _b.a 被调用this 设置为全局对象(或者如果启用了严格模式，则设置为 undefined).如果您要直接调用 _b.a()，则在调用 _b.a 时将 this 设置为 _b>.

(0, _b.a)() ensures that the function _b.a is called with this set to the global object (or if strict mode is enabled, to undefined). If you were to call _b.a() directly, then _b.a is called with this set to _b.

(0, _b.a)(); 等价于

0; // Ignore result
var tmp = _b.a;
tmp();

(, 是逗号操作符，参见 https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Comma_Operator).

(the , is the comma operator, see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Comma_Operator).

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