How to override node.js core module? - node.js

Question

I would like to use https://github.com/isaacs/readable-stream instead of core node.js stream module. However I would like other third-party modules to use it as well - is it possible to override core module at runtime?
For example I would like this code:
var stream = require('stream');
to return readable-stream library instead of core stream module.

Answer 1

You can use the following module (mock-require.js):
'use strict';
var Module = require('module');
var assert = require('assert');
var require = function require(path) {
assert(typeof(path) == 'string', 'path must be a string');
assert(path, 'missing path');
var _this = this;
var next = function() { return Module.prototype.require.next.apply(_this, arguments); };
console.log('mock-require: requiring <' + path + '> from <' + this.id + '>');
switch (path) {
case 'stream':
// replace module with other
if (/\/readable-stream\//.exec(this.filename)) {
// imports from within readable-stream resolve into original module
return next('stream');
} else {
return next('readable-stream');
}
case 'events':
// mock module completely
return {
EventEmitter: next('eventemitter2').EventEmitter2,
usingDomains: false
}
case 'hello/world :)':
// name can be anything as well
return { hello: 'world!' };
default:
// forward unrecognized modules to previous handler
console.log(path);
return next(path);
}
};
require.next = Module.prototype.require;
Module.prototype.require = require;
module.exports = {};
You need to require it once somewhere in your project, so require() is properly intercepted. You are also free to provide some flexible API to registering/unregistering mock modules (something like require.filter(/^foo\/\d+/, function(path) { return { boo: 'hoo' }; }); — I hadn't bothered with it yet.
Usage example then is as follows:
'use strict';
require('./mock-require');
require('util');
console.log(require('hello/world :)'));
console.log(require('events'));
console.log(require('stream'));