Is module.exports equal exports

Question

Im a newcomer on nodejs, try to know how include function from other files.

I found module.exports and exports can sovle my problem, but I dont know what's different between the two.

I do some research, and found exports = module.exports
ref: Understanding module.exports and exports in Node.js

var exports = module.exports = {};

But I still not sure of that, so I do the experiment :

index.js

const lib1 = require('./lib1.js');
const lib2 = require('./lib2.js');
const lib3 = require('./lib3.js');
const lib4 = require('./lib4.js');

const func = () => {
    return 'attr2';
};

var lib = {};
var lib = {
    attr1: 'attr1',
    attr2_1: func,
    attr2_2: func(),
    attr3: () => {
        return 'attr3 (closure)';
    }
};

// using object variable on local file
console.log(lib.attr1);
console.log(lib.attr2_1());
console.log(lib.attr2_2);
console.log(lib.attr3());

console.log('# Case1 ----');

console.log(lib1.attr1);
console.log(lib1.attr2_1());
console.log(lib1.attr2_2);
console.log(lib1.attr3());

console.log('# Case2 ----');

console.log(lib1.attr1);
console.log(lib1.attr2_1());
console.log(lib1.attr2_2);
console.log(lib1.attr3());

console.log('# Case3 ----');

console.log(lib3.attr1);
console.log(lib3.attr2_1());
console.log(lib3.attr2_2);
console.log(lib3.attr3());

console.log('# Case4 ----');

console.log(lib4.attr1);
console.log(lib4.attr2_1());
console.log(lib4.attr2_2);
console.log(lib4.attr3());

lib1.js

const func = () => {
    return 'attr2';
}

// Case1 : This work
module.exports.attr1 = 'attr1';
module.exports.attr2_1 = func;
module.exports.attr2_2 = func();
module.exports.attr3 = () => {
    return 'attr3 (closure)';
};

lib2.js

const func = () => {
    return 'attr2';
}

// Case2 : This work
module.exports = {
    attr1: 'attr1',
    attr2_1: func,
    attr2_2: func(),
    attr3: () => {
        return 'attr3 (closure)';
    }
};

lib3.js

const func = () => {
    return 'attr2';
}

// Case3 : This work
exports.attr1 = 'attr1';
exports.attr2_1 = func;
exports.attr2_2 = func();
exports.attr3 = () => {
    return 'attr3 (closure)';
};

lib4.js

const func = () => {
    return 'attr2';
}

// Case4 : Not work!
exports = {
    attr1: 'attr1',
    attr2_1: func,
    attr2_2: func(),
    attr3: () => {
        return 'attr3 (closure)';
    }
};

case4 will return error : (from vscode debug)

...

# Case4 ----
undefined
C:\Users\Scott\Documents\module_export\index.js:49
console.log(lib4.attr2_1());
                 ^

TypeError: lib4.attr2_1 is not a function
    at Object.<anonymous> (C:\Users\Scott\Documents\module_export\index.js:49:18)
    at Module._compile (internal/modules/cjs/loader.js:689:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:700:10)
    at Module.load (internal/modules/cjs/loader.js:599:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:538:12)
    at Function.Module._load (internal/modules/cjs/loader.js:530:3)
    at Function.Module.runMain (internal/modules/cjs/loader.js:742:12)
    at startup (internal/bootstrap/node.js:282:19)
    at bootstrapNodeJSCore (internal/bootstrap/node.js:743:3)

case1, 2, 3 => is working fine.

but why case4 is error? is the var exports = module.exports = {}; not true?

Answer 1

What you're doing here

var exports = module.exports = {};
exports = {
    attr1: 'attr1',
    attr2_1: func,
    attr2_2: func(),
    attr3: () => {
        return 'attr3 (closure)';
    }
};

is reassigning the variable named exports to an entirely new object. The variable named exports no longer points to the same object as module.exports ; module.exports remains unchanged, as the empty object. So, when other modules try to import from lib4 , they'll just get an empty object.

You must mutate the exports variable, like you're doing in case 3, or you must reassign the exports property on module , like you're doing in case 2 (or mutate the existing exports object in case 1).

This doesn't particularly have anything to do with imports/exports: the same behavior can be seen here:

 // boilerplate, so that the live snippet will run const module = {}; var exports = module.exports = {}; const func = () => { return 'attr2'; } // Case4 : Not work! exports = { attr1: 'attr1', attr2_1: func, attr2_2: func(), attr3: () => { return 'attr3 (closure)'; } }; // module.exports is still the empty object: console.log(module.exports.attr1); 

Or here:

 var obj1 = {}; var obj2 = obj1; obj2 = { foo: 'foo' }; // reassigning obj2 does not affect obj1 console.log(obj1.foo); // undefined, obj1 is just an empty object, after all 

Unless module.exports somehow gets populated, nothing will be exported.

Answer 2

Like you mentioned, exports is a reference to module.exports . When a module is imported, the value in module.exports is returned.

By assigning exports to another object, you are not channging the value of module.exports and hence this error.

Answer 3

In node js, exports is a getter/setter. Once you try getting exports value ( exports ), it will return module.exports. But if you try setting exports value( exports = something ), it will change value of module.exports.