rewire

Dependency injection for node.js applications.

rewire allows you to modify the behaviour of modules for better unit testing. You may

• introduce mocks for other modules
• leak private variables
• override variables within the module
• inject your own scripts

rewire does not load the file and eval the contents to emulate node's require mechanism. In fact it uses node's own require to load the module. Thus your module behaves exactly the same in your test environment as under regular circumstances (except your modifications).

Debugging is fully supported.

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Installation

npm install rewire

For older node versions:
rewire is tested with node 0.6.x. I recommend to run the unit tests via mocha in the rewire-folder before using rewire with older node versions.

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Examples

var rewire = require("rewire");

// rewire acts exactly like require when omitting all other params
rewire("./myModuleA.js") === require("./myModuleA.js"); // = true

Mocks

// You can introduce your own mocks for modules that are required:
rewiredModule = rewire("./myModuleA.js", {
    "fs": {
        readFile: function (path, encoding, cb) { cb(null, "Success!"); }
    },
    "../path/to/moduleB.js": myMockForModuleB
});
// The rewired module will now use your mocks instead of fs
// and moduleB.js. Just make sure that the path is exactly as
// in myModuleA.js required.

Injections

// You can inject your own mocks for internal or global objects.
// These injections are only visible within the module.
rewiredModule = rewire("./myModuleA.js", null, {
    console: {
        log: function () { /* be quiet */ }
    },
    process: { argv: ["some", "other", "args"] },
    __filename: "some/other/dir"
});

// This will inject
// var console = {log: function () { /* be quiet */ }};
// var process = {argv: ["some", "other", "args"]};
// var __filename = "some/other/dir";
// at the end of the module.

// You can also pass a script to inject at the end
rewiredModule = rewire("./myModuleA.js", null, "console.log('hello');");
// This will print "hello" when the module loads

Leaks

// You can expose private variables for unit tests
rewiredModule = rewire("./myModuleA.js", null, null, ["myVar1", "myVar2");

// This will inject
// module.exports.__ = {myVar1: myVar1, myVar2: myVar2}
// at the end of the module.

// You can access now your private variables under the special.__-object
console.log(rewiredModule.__.myVar1);
console.log(rewiredModule.__.myVar2);

Cache

// You can disable caching of the rewired module. Any require()-calls will
// now return the original module again instead of the rewired.
// Caching is enabled by default.
rewire("./myModuleA.js", null, null, null, false) === require("./myModuleA.js");
// = false

// You can also delete all rewired modules from the cache by one call.
rewire.reset();
// You should call this after every unit test to ensure a clean test environment.

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##API

rewire(filename, mocks, injections, leaks, cache)

• {!String} filename:
  Path to the module that shall be rewired. Use it exactly like require().

• {Object} mocks (optional):
  An object with module mocks. Keys should reflect the required path of the module.

• {Object|String} injections (optional):
  If you pass an object, all keys of the object will be vars within the module. You can also eval a string.

• {Array<String>} leaks (optional):
  An array with variable names that should be exported. These variables are accessible via myModule.__.

• {Boolean=true} cache (optional):
  Indicates whether the rewired module should be cached by node so subsequent calls of require() will return the rewired module. Further calls of rewire() will always overwrite the cache.

Returns the rewired module.

rewire.reset()

Removes all rewired modules from require.cache. Every require() will now return the original module again.

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Please note

Keys should be exactly the same like they're required in the target module

So if you write require("../../myModules/myModuleA.js") you need to pass {"../../myModules/myModuleA.js": myModuleAMock}.

All scripts are injected at the end of the module

So if there is any code in your module that is executed during require(), your injected variables will be undefined at this point.

Imagine rewire("./myModule.js", null, {console: null});:

console.log("Hello");   // ouch, that won't work. console is undefined at this point because of hoisting

// End of module ///////////////
// rewire will inject here
var console = null;

Leaks are executed at the end of the module.

All variables, that are copied by value will not be updated anymore.

A good approach to solve this would be:

var myLeaks = {};

module.exports = function (someValue) {
   myLeaks.someValue = someValue;
};

And then: rewire("myModuleA.js", null, null, ["myLeaks"]);

Because myLeaks is defined at the end of the module, you're able to access the leak object and all leaks that are attached to it later during runtime.

Call rewire.reset() after every unit test

All require()s will now return the original module again.

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Credits

This module is inspired by the great injectr-module.

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License

(The MIT License)

Copyright (c) 2012 Johannes Ewald <[email protected]>

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the 'Software'), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED 'AS IS', WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.