Node.js – Understanding Node.js modules: multiple requires return the same object

node.js

I have a question related to the node.js documentation on module caching:

Modules are cached after the first time they are loaded. This means
(among other things) that every call to require('foo') will get
exactly the same object returned, if it would resolve to the same
file.

Multiple calls to require('foo') may not cause the module code to be
executed multiple times. This is an important feature. With it,
"partially done" objects can be returned, thus allowing transitive
dependencies to be loaded even when they would cause cycles.

What is meant with may?

I want to know if require will always return the same object. So in case I require a module A in app.js and change the exports object within app.js (the one that require returns) and after that require a module B in app.js that itself requires module A, will I always get the modified version of that object, or a new one?

// app.js

var a = require('./a');
a.b = 2;
console.log(a.b); //2

var b = require('./b');
console.log(b.b); //2

// a.js

exports.a = 1;

// b.js

module.exports = require('./a');

Best Answer

If both app.js and b.js reside in the same project (and in the same directory) then both of them will receive the same instance of A. From the node.js documentation:

... every call to require('foo') will get exactly the same object returned, if it would resolve to the same file.

The situation is different when a.js, b.js and app.js are in different npm modules. For example:

[APP] --> [A], [B]
[B]   --> [A]

In that case the require('a') in app.js would resolve to a different copy of a.js than require('a') in b.js and therefore return a different instance of A. There is a blog post describing this behavior in more detail.

Related Solutions

Javascript – How to get a list of the names of all files present in a directory in Node.js

You can use the fs.readdir or fs.readdirSync methods. fs is included in Node.js core, so there's no need to install anything.

fs.readdir

const testFolder = './tests/';
const fs = require('fs');

fs.readdir(testFolder, (err, files) => {
  files.forEach(file => {
    console.log(file);
  });
});

fs.readdirSync

const testFolder = './tests/';
const fs = require('fs');

fs.readdirSync(testFolder).forEach(file => {
  console.log(file);
});

The difference between the two methods, is that the first one is asynchronous, so you have to provide a callback function that will be executed when the read process ends.

The second is synchronous, it will return the file name array, but it will stop any further execution of your code until the read process ends.

Node.js – How to get the path to the current script with Node.js

I found it after looking through the documentation again. What I was looking for were the __filename and __dirname module-level variables.

__filename is the file name of the current module. This is the resolved absolute path of the current module file. (ex:/home/kyle/some/dir/file.js)
__dirname is the directory name of the current module. (ex:/home/kyle/some/dir)

Best Answer

Related Solutions

Javascript – How to get a list of the names of all files present in a directory in Node.js

Node.js – How to get the path to the current script with Node.js

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