patch-module

Monkey patch npm modules.

Have you ever looked through a module you are using and wished it was written just a little bit different?

Knowing the hassle of forking a repo, changing the code, submitting a Pull Request (which may not get accepted) and then losing track of updates in the original module; I built Patch Module.

 Patch Module allows you to change the source code of another module before it is required. It mimics node's require functionality, making it familiar to use whilst avoiding the use of eval. Patch Module will load a module from the file system, apply the patches you wish to make and then compiles the module.

Patch Module includes a couple of optional safety features too. You can define the version of the module you are patching as well as how many times the regex for the patch should match. If either of those operations don't match your settings, Patch Module will alert you letting you know that the module you are trying to patch has been updated and you should check your patch is still necessary.

Install

$ npm install patch-module --save

Usage

We will use the module is-it-friday as an example of a module to patch. is-it-friday is simply one line module.exports = "Probably not...";

Default is-it-friday behaviour:

// file: index.js
var isItFriday = require('is-it-friday');
console.log(isItFriday); // Probably not...

Patching as a require:

// file: index.js
var patch = require('patch-module');
var isItFriday = patch('./node_modules/is-it-friday/index.js', {
	version: '0.1.0',
	package: './node_modules/is-it-friday/package.json'
}, [
	{find: '"Probably not..."', replace: '(new Date()).getDay() === 5 ? "Yes" : "No"', expect: 1}
]);
console.log(isItFriday); // "Yes" or "No" depending if it's Friday


Here we create our patched isItFriday module so we can require it elsewhere in our project whilst making it easy to revert to the original module later if needed:

// file: index.js
var isItFriday = require('./patch/is-it-friday.js');
console.log(isItFriday); // "Yes" or "No" depending if it's Friday
 
// file: patch/is-it-friday.js
var patch = require('patch-module');
module.exports = patch('./node_modules/is-it-friday/index.js', {
	version: '0.1.0',
	package: './node_modules/is-it-friday/package.json'
}, [
	{find: '"Probably not..."', replace: '(new Date()).getDay() === 5 ? "Yes" : "No"', expect: 1}
]);

Advanced Usage

To show an example of patching multiple files in a module we will use jugglingdb as an example. Let's patch the way jugglingdb handles connection errors when trying to connect to a Mongo database that is not available, by adding the ability to listen to a new error event and handle it instead of it throwing an uncatchable error.

In jugglingdb if a full path is specified for a Schema it will load that instead of looking in node_modules so we can point it directly to our modified jugglingdb-mongodb module.

// file: index.js
var jugglingdb = require('./patch/jugglingdb.js');
var Schema = jugglingdb.Schema;
var db = new Schema(__dirname + '/patch/jugglingdb-mongodb.js', {
	url: process.env.MONGO_URL || 'mongodb://localhost:27017/test'
});
db.on('connected', function () {
	console.log('connected!');
});
db.on('disconnected', function () {
	console.log('disconnected!');
});
// This is new functionality
db.on('error', function (err) {
	console.log('error!', err.message);
});

We need to modify the Schema class, which is loaded from the modules index.js.

Important: If you are modifying a relative require (require(./) you need to replace the new path with the full path.

// file: patch/jugglingdb.js
var patch = require('patch-module');
module.exports = patch('./node_modules/jugglingdb/index.js', {
	version: '2.0.0-rc8',
	package: './node_modules/jugglingdb/package.json'
}, [
	{find: 'var Schema = exports.Schema = require(\'./lib/schema\').Schema;', replace: 'var Schema = exports.Schema = require(\'' + __dirname + '/patch/jugglingdb-schema.js\').Schema;', expect: 1}
]);

Here we are modifying the jugglingdb-mongodb connector module and changing the throw to calling the callback and passing the error on, that way we can choose how to handle it. We expect to make two replacements, so we've set expect to 2.

// file: patch/jugglingdb-mongodb.js
var patch = require('patch-module');
module.exports = patch('./node_modules/jugglingdb-mongodb/lib/mongodb.js', {
	version: '0.2.0',
	package: './node_modules/jugglingdb-mongodb/package.json'
}, [
	{find: /if \(err\) throw err;/mg, replace: 'if (err) return callback(err);', expect: 2},
]);

When jugglingdb connects to a database, it creates a client variable that holds the connection to the database in the connector and passes it back. However it doesn't check if the client variable actually has a valid connection that emits the connected event. So we are checking if the client variable has a valid connection to a database and only emitting the connected event then, if not we will emit a new error event with the error.

// file: patch/jugglingdb-schema.js
var patch = require('patch-module');
module.exports = patch('./node_modules/jugglingdb/lib/schema.js', {
	version: '2.0.0-rc8',
	package: './node_modules/jugglingdb/package.json'
}, [
	{find: 'adapter.initialize(this, function () {', replace: 'adapter.initialize(this, function (err) {', expect: 1},
	{find: /this\.connecting = false;\s+this\.connected = true;\s+this\.emit\('connected'\);/m, replace: `if (this.client) {
			this.connecting = false;
			this.connected = true;
			this.emit('connected');
		} else {
			this.emit('error', err);
		}`, expect: 1}
]);

patch(filePath, options = {}, replacements = [])

• The first argument filePath is the path to the file you want to patch.
• The second argument can be either an options Object, replacements Array, callback Function or undefined.
• The third argument can be a replacements Array, callback Function or undefined.

options Object

• version - Version that we expect from the package file for the module we are modifying
• package - Package file to look in when version is defined
• dontReplaceRelativeRequires - Don't automatically replace require(./ with require(path/to/file/you/are/changing. The module is set to replace relative requires from a module back to the original path by default as a convenience
• callback - callback Function, see callback Function below
• returnAsString - Useful for debugging your modifications, will return the source as a string which you can write to a file or console.log

replacements Array

An Array of replacement Objects with keys:

• find - String or RegExp to look for
• replace - String to replace what we find with
• expect - (Optional) Explicitly define an Integer for how many occurrences of find we expect. This helps with module updates and old patches

callback Function

A Function that takes a string function (str) { /* ...modify str... */ return str; }, modifies it and returns it to be compiled. If you have a callback Function and replacements Array the callback Function modifies the source first than the replacements are applied.

Tests

To run the test suite, first install the dependencies, then run npm test:

$ npm install
$ npm test

License

MIT