You should know that Javascript (better to say EcmaScript) does not specify any function to read and write files.

In fact, Javascript is just the language used by many environments (the browser, or NodeJS, are examples of environments) that offer more objects and functions to work with.

Node was the first environment to offer a way to organize code in modules by using a special function called require(). How does it work? Let's try to implement it from zero.

Here is an example of require at work:

//test.js
module.exports = "Hello World";

//main.js
const test = require("./test.js"); 
console.log(test) 

Let's write that require function.

What should a require() function do

a require function is expected to:

• read the content of a javascript file in a string
• evaluate that code
• save the exported function/object in a cache for later use (only read files once)

Disclaimer

We will not rebuild the whole NodeJS in a single post. In fact, I will not implement many NodeJS checks and giggles, we are only interested in understand how things work.

We will still need the real require function to load the fs module. I'm not cheating, it's just that this post has to end sooner or later :)

myRequire() function

here's the code:

//file setup.js

const fs = require('fs');

myRequire.cache = Object.create(null); //(1)

function myRequire(name) {   
    if (!(name in myRequire.cache)) {     
        let code = fs.readFileSync(name, 'utf8'); //(2)
        let module = {exports: {}}; //(3)
        myRequire.cache[name] = module; //(4)    
        let wrapper = Function("require, exports, module", code); //(5)
        wrapper(myRequire, module.exports, module); //(6)
    }
    return myRequire.cache[name].exports; //(7)
}

...

Did you forget to declare myRequire variable?

No. In Javascript, functions declared with function keyword are evaluated before any other code (functions are "hoisted") so they can be referenced even before they're declared.

Also, functions can have properties (this is javascript!) so you can add the cache property to the myRequire function (step 1).

Finally we're creating the cache property with Object.create. With this function we can specify the object prototype, we have chosen to not specify a prototype. Why? This way we don't mess with other functions or properties declared by the runtime. Here's an explanation.

Let's go back to myRequire . If the file we're importing is not in cache, we read the file from disk (step 2).

Then we declare an empty module object with just one property, exports (step 3).

We add this empty module to the cache, using the filename as the key, and then the magic happens (step 4).

The Function constructor

In JS we can evaluate a string js code in two ways. The first way is via eval() function, that is a bit dangerous (it messes up the scope) so it is highly discouraged to use it.

The second way to evaluate code that we have in a string is via the Function constructor. This constructor takes a string with the arguments and a string with the code. This way everything has its own scope and doesn't mess things up for others.

So, basically we are creating a new function with these variables (step 5): require, exports, and module. Let's think for a moment at the first example of this post, the file test.js: it becomes

function(require, exports, module) {
  module.exports = "Hello World" 
}

and the second file, main.js:

function(require, exports, module) {
  const test = require("./test.js"); 
  console.log(test) 
}

Variables that seemed "global" in files are indeed passed as function arguments.

Last step: executing the function

We have created (step 6) a wrapper variable that holds a function, but the function is never executed. We do this at the line:

wrapper(myRequire, module.exports, module); 

Note that the second variable (that should be exports) is just a handle to module.exports; NodeJS creators thought that this could have helped in writing less code...

When Node executes the function, everything that was "exported" (your public API) gets linked to the cache.

(Remember the myRequire.cache[name] = module; line? When it was first found by the compiler it was point to a dummy { exports: {} } object; now it contains your module.)

NOTE. Since we pass myRequire to the wrapper function, we can from now on use require in our test files, but our require gets called. Add a console.log if you don't trust me ;)

Finally... myRequire returns the exported stuff you declared (step 7), and that we saved to the cache so we won't have to reevaluate this code again.

Final considerations

An example of this code can be found here, along with some console logs that explain what's going on.

The idea of this article comes from the explanation of this function at chapter 10 (Modules). The book (Eloquent Javascript) is excellent, but I had the urge to better understand, and try with a debugger, what I could not understand with my mind alone.

You should definitely read the book if you want to better understand javascript.