Requizzle

Swizzle a little something into your Node.js modules.

What's Requizzle?

Requizzle provides a drop-in replacement for Node.js's require() function. This replacement enables you to change a module's source code when Node.js loads the module.

You can use Requizzle in your test cases, or in production code if you like to live dangerously.

How can I change a module with Requizzle?

There are several different ways:

Look for modules in new places

With Requizzle, you can add directories to the module lookup path, which forces Node.js to search those directories for modules. This can be useful if:

• You're tired of writing code like require('../../../../../lib/foo').
• You want to expose your app's modules to external plugins.

Add code before or after the module's source code

Tamper with modules to your heart's delight by adding arbitrary code before or after the module's own source code.

Mess with child modules

When you use Requizzle to require a module, you can force each child module's require method to inherit your changes to the parent module. (By default, only the parent module is changed.)

Will Requizzle break my dependencies?

Probably not. It's true that Requizzle gives you plenty of new and exciting ways to tamper with, and possibly break, your module dependencies. But Requizzle also tries not to break anything on its own. In particular:

• Requizzle preserves strict-mode declarations. If a module starts with a strict-mode declaration, Requizzle keeps it in place. Your changes will appear after the strict-mode declaration.
• Requizzle leaves native modules alone. If you use Requizzle to load one of Node.js's built-in modules, such as fs or path, Requizzle won't mess with it.

Usage

The Requizzle module exports a single function, which returns a drop-in replacement for require().

When you call the function, you must pass in an options object, which can include any of these properties:

• extras: A pair of functions that return text to insert before or after the module's source code. Each function accepts two parameters: targetPath, the path to the required module, and parentModule, the Module object for the module's parent. Each function must return a string.

  • extras.before: A function that returns text to insert before the module's source code.
  • extras.after: A function that returns text to insert after the module's source code.

• infect: Determines whether child modules are infected with the same changes as the parent module. Set to true to force child modules to inherit your changes. Defaults to false.

• requirePaths: Additional paths to search for required modules. For example, if requirePaths is set to ['/usr/lib/junk/modules'], and you save a JavaScript module at /usr/lib/junk/modules/mymodule.js, you can require the module as mymodule.

  You can provide an array of paths, which will be searched before the default module paths, or an object with the following properties:

  • requirePaths.before: An array of paths to search before the default module paths.
  • requirePaths.after: An array of paths to search after the default module paths. Use this property if you want the module to use its own local dependencies when possible, then fall back to the additional paths if necessary.

  By default, the require path is not changed.

Examples

const requizzle = require('requizzle');

// Say hello and goodbye to each module.
const logRequire = requizzle({
  extras: {
    before: function(targetPath, parentModule) {
      return 'console.log("Hello %s!", ' + targetPath + ');\n';
    },
    after: function(targetPath, parentModule) {
      return 'console.log("Goodbye %s!", ' + targetPath + ');\n';
    }
  }
});
// Prints "Hello /path/to/mymodule.js!" and "Goodbye /path/to/mymodule.js!"
const myModule = logRequire('mymodule');

// Look for modules in the current module's `lib` directory, and force child
// modules to do the same.
const path = require('path');
const extraPathRequire = requizzle({
  infect: true,
  requirePaths: [path.join(__dirname, 'lib')]
});
// If `foo` needs to require a module in `./lib`, it can use `require('bar')`
// instead of `require('./lib/bar')`.
const foo = extraPathRequire('./foo');

Troubleshooting

Here are some problems you might run into when you use Requizzle, along with solutions to each problem. If you run into any problems that aren't addressed here, please file a new issue!

Requizzle slowed down my code! A lot!

Requizzle adds minimal overhead to the module-loading process. However, your code will run much slower than usual if you do both of the following:

• Use Requizzle's infect option.
• Require modules that have a lot of require() calls within the scope of individual functions.

If Requizzle seems to slow down your app, look for module calls that are within function scope, then move them to each module's top-level scope.

Requizzle made my module do something weird!

Do you have any circular dependencies in the modules that aren't working? Circular dependencies can cause unusual behavior with Requizzle, just as they can without Requizzle. Try breaking the circular dependency.

Requizzle violates the Law of Demeter! It's an unnatural abomination!

Fair enough.

Changelog

• 0.2.4 (November 2022): Fixed a compatibility issue with core modules that are loaded with the node: prefix, as in require('node:fs').
• 0.2.3 (July 2019): Updated dependencies.
• 0.2.2 (May 2019): Fixed a compability issue with Node.js 12.
• 0.2.1 (December 2014): The requirePaths option no longer inserts an extra line break into the source file.
• 0.2.0 (June 2014): The requirePaths option can now contain before and after properties. Paths in the before property will be searched first; paths in the after property will be searched last.
• 0.1.1 (June 2014): If the requirePaths option is used, the module loader now searches the extra paths first rather than last.
• 0.1.0 (June 2014): Initial release.

Acknowledgements

Requizzle is very loosely adapted from Johannes Ewald's rewire module, which is designed to modify a module's behavior for unit testing. If Requizzle doesn't meet your needs, please take a look at rewire!

License

MIT license.