rewire

What is rewire?

Rewire is a testing utility for Node.js that allows you to modify the behavior of a module by overriding its private variables and functions. This is particularly useful for unit testing, as it enables you to test private functions and mock dependencies.

What are rewire's main functionalities?

Accessing Private Variables

This feature allows you to access private variables within a module. In the code sample, `privateVar` is a private variable in `myModule` that is accessed using `__get__`.

const rewire = require('rewire');
const myModule = rewire('./myModule');
const privateVar = myModule.__get__('privateVar');
console.log(privateVar);

Mocking Private Functions

This feature allows you to mock private functions within a module. In the code sample, `privateFunction` is a private function in `myModule` that is mocked using `__set__`.

const rewire = require('rewire');
const myModule = rewire('./myModule');
myModule.__set__('privateFunction', function() { return 'mocked'; });
console.log(myModule.somePublicFunction());

Mocking Dependencies

This feature allows you to mock dependencies of a module. In the code sample, `dependency` is a dependency of `myModule` that is mocked using `__set__`.

const rewire = require('rewire');
const myModule = rewire('./myModule');
myModule.__set__('dependency', { someMethod: () => 'mocked' });
console.log(myModule.somePublicFunction());

Other packages similar to rewire

proxyquire

Proxyquire is a module that allows you to override dependencies during testing. Unlike Rewire, which focuses on accessing and modifying private variables and functions, Proxyquire is more about replacing entire dependencies with mocks or stubs.

sinon

Sinon is a testing library that provides standalone test spies, stubs, and mocks. While it doesn't allow you to access private variables directly like Rewire, it is very powerful for creating mocks and stubs for functions and objects.

testdouble

Testdouble is a library for creating test doubles (mocks, stubs, etc.). It focuses on providing a clean and simple API for creating test doubles, but it doesn't provide the ability to access or modify private variables like Rewire.

README

rewire

Dependency injection for node.js applications.

rewire adds a special setter and getter to modules so you can modify their behaviour for better unit testing. You may

• introduce mocks for other modules
• leak private variables
• override variables within the module.

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.

Furthermore rewire comes also with support for browserify. Thus you can mock your modules in the browser as well.

Installation

npm install rewire

For older node versions:
rewire is tested with node 0.6.x - 0.7.x. I recommend to run the unit tests via mocha in the rewire-folder before using rewire with older node versions.

Use with browserify:

// debug=true splits the source in seperate files in chrome's developer tools
var b = require("browserify")({debug: true});

b.use(require("rewire").browserify);

Examples

var rewire = require("rewire");


// rewire acts exactly like require.
var myModule = rewire("../lib/myModule.js");


// Your module will now export a special setter and getter for private variables.
myModule.__set__("myPrivateVar", 123);
myModule.__get__("myPrivateVar"); // = 123


// This allows you to mock almost everything within the module e.g. the fs-module.
// Just pass the variable name as first parameter and your mock as second.
myModule.__set__("fs", {
    readFile: function (path, encoding, cb) {
        cb(null, "Success!");
    }
});
myModule.readSomethingFromFileSystem(function (err, data) {
    console.log(data); // = Success!
});


// All later requires will now return the module with the mock.
myModule === require("./myModule.js"); // = true


// You can set different variables with one call.
myModule.__set__({
    fs: fsMock,
    http: httpMock,
    someOtherVar: "hello"
});


// You may also override globals. These changes are only within the module, so
// you don't have to be concerned that other modules are influenced by your mock.
myModule.__set__({
    console: {
        log: function () { /* be quiet */ }
    },
    process: {
        argv: ["testArg1", "testArg2"]
    }
});


// But be careful, if you do something like this you'll change your global
// console instance.
myModule.__set__("console.log", function () { /* be quiet */ });


// By getting private variables you can test for instance if your
// module is in a specific state
assert.ok(myModule.__get__("currentState") === "idle");


// You can also disable caching when loading the rewired module. All
// subsequent calls of require() will than return the original module again.
rewire("./myModule.js") === require("./myModule.js"); // = true
rewire("./myModule.js", false) === require("./myModule.js"); // = false


// Every call of rewire returns a new instance and overwrites the old
// one in the module cache.
rewire("./myModule.js") === rewire("./myModule.js"); // = false


// If you want to remove all your rewired modules from the
// cache just call rewire.reset().
// Do this before every unit test to ensure a clean testing environment.
rewire.reset();

##API

rewire(filename, cache): {RewiredModule}

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

• {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.

rewire.reset()

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

RewiredModule.__set__(name, value)

• {!String} name:
  Name of the variable to set. The variable should be a global or defined with var in the top-level scope of the module.

• {∗} value:
  The value to set

RewiredModule.__set__(env)

• {!Object} env:
  Takes all keys as variable names and sets the values respectively.

RewiredModule.__get__(name): {∗}

Returns the private variable.

Credits

This module is inspired by the great injectr-module.