javascript - Object.assign(module.exports, {...}) 与 module.exports = {...}

Question

有人可以用示例解释 module.exports = {...} 将如何导致意外行为。

我读到你还不知道 js，我在 https://github.com/getify/You-Dont-Know-JS/blob/2nd-ed/scope-closures/ch8.md#node-commonjs-modules

Some developers have the habit of replacing the default exports object, like this:

// defining a new object for the API
module.exports = {
    // ..exports..
};

There are some quirks with this approach, including unexpected behavior if multiple such modules circularly depend on each other. As such, I recommend against replacing the object. If you want to assign multiple exports at once, using object literal style definition, you can do this instead:

Object.assign(module.exports,{
   // .. exports ..
});

What's happening here is defining the { .. } object literal with your module's public API specified, and then Object.assign(..) is performing a shallow copy of all those properties onto the existing module.exports object, instead of replacing it This is a nice balance of convenience and safer module behavior.

Answer 1

exports 对象是在模块运行之前为您的模块创建的，并且如果存在循环依赖关系，则其他模块可能会先于您的模块访问该默认对象。填写它。如果您替换它，他们可能拥有旧的原始对象，并且(最终)看不到您的导出。如果您添加到它，那么即使该对象最初没有您的导出，最终它也会拥有它，即使其他模块在这些导出存在之前访问了该对象。

更多内容请参见 Cycles section CJS 模块文档。

我们可以调整该部分中的循环示例来演示它:

a.js(注意更改):

console.log('a starting');
// Removed: `exports.done = false;`
const b = require('./b.js');
console.log('in a, b.done = %j', b.done);
exports = {done: true}; // ** Modified this line
console.log('a done');

b.js(不变):

console.log('b starting');
exports.done = false;
const a = require('./a.js');
console.log('in b, a.done = %j', a.done);
exports.done = true;
console.log('b done');

main.js(不变):

console.log('main starting');
const a = require('./a.js');
const b = require('./b.js');
console.log('in main, a.done = %j, b.done = %j', a.done, b.done);

当你运行它时:

main starting
a starting
b starting
in b, a.done = undefined
b done
in a, b.done = true
a done
in main, a.done = undefined, b.done = true
(node:2025) Warning: Accessing non-existent property 'done' of module exports inside circular dependency
(Use `node --trace-warnings ...` to show where the warning was created)

---

旁注:循环的处理方式与 JavaScript 自己的模块系统 (ESM) 不同，并且由于没有 exports 等效对象(您可以访问；概念上存在)，因此不会出现此问题。我建议尽可能使用 ESM。自 v12 以来，Node.js 就以相当稳定(尽管仍在不断发展)的方式支持它。