Sunday, 14 October 2018

What is the purpose of Node.js module.exports and how do you use it?

Background

If you have ever used nodejs and npm you must have encountered usage of module.exports syntax. Or perhaps just exports? If you do have encountered this that you might be aware that it exposes a certain functionality of the code so that other outside code can reference and execute it. Though this is a short version of what exports syntax does, in this post we will see a lot more of how it behaves and some examples to help us understand this better.










What is the purpose of Node.js module.exports and how do you use it?

The module.exports object is created by the Module system. module.exports is the object that's actually returned as the result of a require call. exports is just an alias to module.exports. So you could use either. There are some things that you would need to take care if you are just using exports but more of that later. For now, let's just take a simple example and see how it actually works.

First, create a file called calc.js  with the following content -

var add = function(x,y) {
return x + y;
}

var subtract = function(x,y){
return x - y;
}

module.exports = {
add  : add,
subtract: subtract
};



This code is simply defining two functions -  add and subtract and exporting them. Notice how these methods are declared as exports. We will revisit this back in a moment. Now let's try to use this code in a separate node code.

Now create another file - Let's call it demo.js. Now in this file add following code -

var calc = require('./calc.js');

console.log("2 + 3 = " + calc.add(2,3));
console.log("3 - 2 = " + calc.subtract(3,2));

And finally, run the demo.js file as -

  • node demo.js
It should give you the expected results -




If you understood above logic you must have got a basic idea of how exports work. To revisit our earlier statement we said - "module.exports is the object that's actually returned as the result of a require call." In this case, it returns a map of add and subtract which point to corresponding functions that you can invoke.

NOTE: Notice the "./" in the require statement. This is required to tell node that it is a local package.

Another way to use the same module.exports would be as follows -

module.exports.add = function(x,y) {
return x + y;
}

module.exports.subtract = function(x,y){
return x - y;
}


And if you now run demo.js again you would still see the same output. Both are just different ways to expose your code outside.

NOTE: Note that assignment to module.exports must be done immediately. It cannot be done in any callbacks. So following will not work -


var add = function(x,y) {
return x + y;
}

var subtract = function(x,y){
return x - y;
}

setTimeout(() => {
  module.exports = { add : add, subtract : subtract };
}, 0);



This will fail with below error -

TypeError: calc.add is not a function
    at Object.<anonymous> (demo.js:3:31)
    at Module._compile (module.js:653:30)
    at Object.Module._extensions..js (module.js:664:10)
    at Module.load (module.js:566:32)
    at tryModuleLoad (module.js:506:12)
    at Function.Module._load (module.js:498:3)
    at Function.Module.runMain (module.js:694:10)
    at startup (bootstrap_node.js:204:16)
    at bootstrap_node.js:625:3



So make sure your exports are done immediately and not in any callbacks.



Now let's come back to the exports keyword that we said is just an alias to module.exports. Let us rewrite the code with just exports now.  Let's say your code is as follows -


exports.add = function(x,y) {
return x + y;
}

exports.subtract = function(x,y){
return x - y;
}


Now run demo.js and you should still get your desired output. Like I said earlier exports is just an alias to exports.module. Now lets out 1st approach with just exports -

var add = function(x,y) {
return x + y;
}

var subtract = function(x,y){
return x - y;
}

exports = {
add : add,
subtract: subtract
}


And if you run demo.js again you will get an error -

TypeError: calc.add is not a function
    at Object.<anonymous> (demo.js:3:31)
    at Module._compile (module.js:653:30)
    at Object.Module._extensions..js (module.js:664:10)
    at Module.load (module.js:566:32)
    at tryModuleLoad (module.js:506:12)
    at Function.Module._load (module.js:498:3)
    at Function.Module.runMain (module.js:694:10)
    at startup (bootstrap_node.js:204:16)
    at bootstrap_node.js:625:3

So why did this happen? This brings us to another crucial fact -

If you overwrite exports then it will no longer refer to module.exports. The exports variable is available within a module's file-level scope, and is assigned the value of module.exports before the module is evaluated. So if you overwrite exports it is no longer an alias to module.exports and hence it no longer works.

So make sure you do not overwrite exports variable. It is always safer to just use module.exports.

To summarize this in a single picture module.exports work as follows -




Related Links

No comments:

Post a comment

t> UA-39527780-1 back to top