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

Question

有人可以举例说明 module.exports = {...} 将如何导致意外行为。

我读到你还不了解 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

在你的模块运行之前为你的模块创建exports对象，并且如果存在循环依赖，其他模块可能可以访问在您的模块可以填充它之前创建默认对象。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就可以访问该对象。

CJS 模块文档的 Cycles section 中有更多内容。

我们可以修改该部分中的循环示例来演示它：

a.js（注意改动）：

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);

当您运行时：

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：JavaScript 自己的模块系统 (ESM) 对循环的处理方式不同，并且由于没有 exports 等效对象（您可以访问；概念上有），这问题不会出现。我建议尽可能使用 ESM。 Node.js 自 v12.

以来以相当稳定（尽管仍在发展）的方式支持它

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

Object.assign(module.exports, {...}) vs module.exports = {...}

javascript

node.js

commonjs