Understanding Require in NodeJs

require in NodeJs is very different from the client-side library RequireJs. The latter will not be covered in this post.

require is an implementation in NodeJs to support moduling following CommonJs specfication.

require works like include (in C++) or import (in Java) things. The basic functionanlity of require is that it reads a javascript file, executes the file and proceeds to return the exports object.

An example of a required file:

console.log('evaluating example.js');
exports.message = 'hello world';

If you run var example=require('./example.js'), we will see evaluating example.js on the console, and example be an object equal to:

{
    message: "hello world"
}

However, you shold not change the reference of exports. An error example:

console.log('Hello Error world');
// exports will NOT be returned.
exports = function Error(){};

Your code can be view in this way:

var module = { exports: {} };
var exports = module.exports;
// Your code
return module.exports;

module is a plain Javascript object. If you change exports by exprots = another_object or exports = a_function, you will lose it (module.exports will not be changed after the reference of exports is changed.)

Some other points you should know about require:

• required exports is cached.
• you can omit .js and require will automatically append it if needed.
• If the file doesn’t start with “./” or “/”, then it is consided as either a core module or located in local node_modules folder.
• If the file stars with “./”, it is considered a relative file to the file that called require.
• If the file starts with “/”, it is consided an absolute path.
• If the file-name is actually a directory, it will first look for package.json and load the file referenced in the main property; otherwise, it will look for an index.js.

References:

1. http://docs.nodejitsu.com/articles/getting-started/what-is-require
2. http://stackoverflow.com/questions/16383795/difference-between-module-exports-and-exports-in-the-commonjs-module-system/16383925?noredirect=1#16383925