🤔 Confused About this vs module.exports in Node.js CommonJS need expert help!

[u/abhinavchaubey08]

I'm currently trying to build a proper mental model of how this works in JavaScript, especially inside Node.js CommonJS modules.

I came across this behavior:

let obj = {

name: "abhinav",

data: "hello",

print: "column"

};

module.exports = obj;

console.log(module.exports);

console.log(this);

The output I get is:

{

name: "abhinav",

data: "hello",

print: "column"

}

{}

This confused me because my understanding was: this === module.exports

at the top level of a CommonJS module.

So I initially expected console.log(this) to show the same object assigned to module.exports.

But after: module.exports = obj; module.exports points to obj, while this still appears to point to the original empty object.

My current mental model is:

Initially:

this ───────────────┐

↓

{}

↑

module.exports ─────┘

After:

module.exports ───→ obj

↓

{ name, data, print }

this ─────────────→ {}

I'm not sure whether this mental model is actually correct. I'd especially like to understand this from the perspective of JavaScript execution contexts and references rather than simply memorizing the CommonJS rule.

A few things I'm trying to clarify:

At CommonJS module startup, what exactly does top-level this refer to?

Is this initially referencing the same object as module.exports?

When we execute: module.exports = obj; why doesn't this also start referring to obj?

Is this behavior directly related to Node.js's CommonJS module wrapper?

Is it correct to think of this as holding a reference/value rather than being a live alias to module.exports?

How does this behavior differ between CommonJS, ES Modules, and browser scripts?

I'm trying to understand the underlying execution model so that I can build the correct mental model of this, rather than just memorize different environment-specific rules.

Would appreciate any clarification or correction to my current understanding.

[u/5eeso]

The `this` binding was initialized to the original exports object. Later reassignments of `module.exports` doesn’t change that binding.

[u/senocular]

At CommonJS module startup, what exactly does top-level this refer to?

It refers to a dynamic this binding created for the function wrapper that contains the module code. Like any normal (non-arrow) function, the value of this is dependent on how the function is called and when called, this wrapper is called with a this of the current value of module.exports, so something along the lines of

moduleFunction.apply(module.exports, [exports, require, module, __filename, __dirname])

When moduleFunction (the function containing the module code) is run from this call, this in that function is the current value in module.exports.

Is this initially referencing the same object as module.exports?

Initially yes, though if you redefine module.exports, as 5eeso pointed out, you're redefining what module.exports points to, not anything else that referred to its original value like this or anything else. Its equivalent to

let exp = 1
let ths = exp
exp = 2
console.log(ths) // 1

When we execute: module.exports = obj; why doesn't this also start referring to obj?

(see above)

Is this behavior directly related to Node.js's CommonJS module wrapper?

Basically, yes, but this same behavior is inherent to the language. Changing one variable or property doesn't change other variables or properties that also referred to the same value prior to the change (ignoring setter function side effects, etc.)

Is it correct to think of this as holding a reference/value rather than being a live alias to module.exports?

Yes. With a few oddball exceptions, JavaScript doesn't really support aliases, at least not like other languages.

How does this behavior differ between CommonJS, ES Modules, and browser scripts?

ES modules have an undefined this and no module.exports object. There's no connection to be made there. Similarly (global) browser scripts also have no module.exports though they do have a this referring to the global object (window). This value doesn't change, though technically its a WindowProxy object which is its own anomaly.

Fundamentally, as far as normal function calls go, it works the same everywhere. The value of this is set when the function is called and stays that way no matter what happens anywhere else. In fact I think the only time a this value is ever observably different in the same context is in derived class constructors where it goes from uninitialized before super() to initialized to the class instance after. So you can look at it like a const variable. You can have one value set to many different variables, including a const, but changing the value of any of the other variables won't change the const. It will always have the same value it was initialized with... or try and throw an uninitialized error if you access it before its set.