Weird behavior with variable scope and 'require' in NodeJS

Question

I have 2 files:

testrequire.js

let a = {};

function foo() {
    a = 'test';
 };

module.exports.foo = foo;
module.exports.a = a;

test.js

let a = require('./testrequire');

a.foo();

console.log(a);

When I run test.js, this is the result:

{ foo: [Function: foo], a: {} }

But I expect it to be like this:

{ foo: [Fuction: foo], a: 'test' }

However, when I change testrequire.js like this:

let a = {};

function foo() {
    a.b = 'test';
};

module.exports.foo = foo;
module.exports.a = a;

The result is:

{ foo: [Function: foo], a: { b: 'test' } }

And it is perfectly like what I expected.

The question here is: Why function foo() can modify a's properties while it cannot modify a?

P/S: I did try var instead of let and the result is still the same. So it is definitely not ES6 let fault.

You have defined a to be an object. Object contains key-value pair. So, try writing a={b: 'test'} in your first example. — Avantika Saini
– Avantika Saini, Commented Feb 27, 2017 at 4:38

slebetman · Accepted Answer · 2017-02-27 04:53:26Z

It's a pointer thing. It's the same in C/C++, Java etc. We've gotten so used to closures that we've sort of expect regular pointers to work the same. But pointers/references are simple indirections.

Let's walk through your code:

let a = {};

Create an object ({}) and point the variable a to that object.

function foo() {
    a = 'test';
};

Declare a function foo() that overwrites the value of a with a string. Now, if you remember your C/assembly then you'd remember that the value of a pointer is the address of the thing it points to. So the original value of a is not {} but the address to that object. When you overwrite a with a string that object still exist and can be garbage collected unless something else points to it.

module.exports.foo = foo;
module.exports.a = a;

Export two properties, 1. foo which points to a function and 2. a which points to the same object that a is pointing to. Remember, just like in C/Java this does not mean that module.exports.a points to a but that it points to {}. Now you have two variables pointing to the same object {}.

Now, when you do:

a.foo();

All you're doing is changing the enclosed variable a to point to a string instead of the original object. You haven't done anything to a.a at all. It's still pointing to {}.

Workarounds

There are two ways to get what you want. First, the OO way. Don't create a closure for a, make it a regular object property:

function foo() {
  this.a = 'test';
};
module.exports.foo = foo;
module.exports.a = {};

This will work as expected because modules in node.js are proper singletons so they can be treated as regular objects.

The second way to do this to use a getter to get the enclosed a. Remember that closures only work with functions, not objects. So just assigning the variable to a property like you did results in a regular pointer operation not a closure. The workaround is this:

let a = {};

function foo() {
  a = 'test';
};
function getA() {
  return a; // this works because we've created a closure
}
module.exports.foo = foo;
module.exports.getA = getA;

Now you can do:

a.foo();
a.getA(); // should return 'test'

Thilo · Accepted Answer · 2017-02-27 04:39:04Z

2

foo can modify the variable a to point to something else.

But this has no effect on the object exported. When the require is done, the calling module receives whatever a pointed to at the time. After that, it does not care about what happens to (the variable) a.

In your second example, you are not assigning a to a new object, but you are modifying the existing object (by adding a new field). That will of course be visible to anyone who got hold of that object before.

This is (very roughly) analogous to

function callerDoesNotSeeThis(a){  a = 1 }

function callerSeesThis(a){  a.foo = 1 }

answered Feb 27, 2017 at 4:39

Thilo

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