2

i want to understand a little bit more about the javascript reference scope, i was trying to import a function from onother file like this

file1.js

exports.checkIfCan = function(){
 //make some check
}

exports.calculate = function(){
   this.checkIfCan();
}

and using the calculate method with this import

file2.js

const { calculate } = require('./file1.js')

calculate()

actually the error is this.checkIfCan is not a function but if i import all the module like this it works

const calculation = require('./file1.js')

calculation.calculate();

I just wanna understand more about the global/local scope in javascript

Improve this question
5
  • You need to check classes instead. You want to create an object with functions on it. Check this: stackoverflow.com/questions/5334383/… Commented Apr 24, 2021 at 13:28
  • 1
    I'm not getting any error and both of your cases are working in my case....lol Commented Apr 24, 2021 at 14:04
  • Is exports.calculate = () => { definitely using an arrow function, and isn't exports.calculate = function() {. Your error seems to suggest that you're not using arrow functions Commented Apr 24, 2021 at 14:08
  • @NickParsons you're right, i'm not using arrow function, but why it should work with? Commented Apr 24, 2021 at 14:24
  • @VladVlads the this keyword works slightly differently in arrow functions than it does within regular functions. Commented Apr 24, 2021 at 14:29

3 Answers 3

Reset to default
2

this refers to the context of the function invocation, which in your case is undefined (on browsers the default is window).

Here's an example as simple as possible to make you understand it:

calculation.calculate(); // <= you are calling calculate from calculation, so 'this' is calculation

calculate(); // <= you are calling calculate from... nothing, so 'this' is undefined

Update: using arrow functions it would work either case because this will be taken from the function's surrounding scope instead of the context, and in the top-level code in a Node module this is equivalent to module.exports.

There's a lot to explain about the thisin javascript, may I suggest you read the free ebook You Don't Know JS: this and Object prototypes? You'll find much more information there.

Improve this answer
Sign up to request clarification or add additional context in comments.

4 Comments

This would be the case if OP was using regular functions, but since OP is using arrow functions the this will be taken from the surrounding scope and not based on how the function is called.
@NickParsons What a fail! You're right, I didn't even read it correctly. I'll either improve my answer or remove it, thanks for pointing it out
no worries. But it looks like OP has updated their answer, so your answer is good now 🙌
That's great, but still I've added some indication about arrow functions and node. Hope it's comprehensible 😬
0

@Vlad Vlads, your problem is in the use of keyword this in your file file file1.js.

In accordance to https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this, following behaviour may happen:

The value of this is determined by how a function is called (runtime binding). It can't be set by assignment during execution, and it may be different each time the function is called.

With that been said, if you remove the keyword this in your function, following will then works as expected:

const { calculate } = require('./file1.js')

calculate()

Your second example will also works as expected

const calculation = require('./file1.js')

calculation.calculate()
Improve this answer

Comments

-1

You should use import instead:

file1.js

function abc() {}

export {
   abc
}

file2.js

import { abc } from 'file1.js';
abc();
Improve this answer

3 Comments

import is ES6 module syntax, which OP isn't using here. require() and module.exports is fine for CommonJS modules, which is what OP appears to be using here.
maybe he's using, you don't know. if he happen to use it in node.js like he tagged or frameworks like React, Vue, or Angular, then using import is supported and it is where the future is heading. You can't just downvote like this.
The exports object does not exist in ES6 modules, so if the OP was using that then they’d be getting other errors. For what it’s worth the down vote isn’t mine

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.