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

• inject 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).

Furthermore rewire comes also with support for various client-side bundlers (see below).

Installation

npm install rewire

Examples

var rewire = require("rewire");


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


// Just with one difference:
// 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!
});


// 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 */ });

// There is another difference to require:
// Every call of rewire() returns a new instance.
rewire("./myModule.js") === rewire("./myModule.js"); // = false

##API

###rewire(filename): rewiredModule

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

###rewiredModule.__set__(name, value)

• name:
  Name of the variable to set. The variable should be global or defined with var in the top-leve scope of the module.
• value:
  The value to set.

###rewiredModule.__set__(env)

• env:
  Takes all keys as variable names and sets the values respectively.

###rewiredModule.__get__(name): value

Returns the private variable.

##Client-Side Bundlers Since rewire relies heavily on node's require mechanism it can't be used on the client-side without adding special middleware to the bundling process. Currently supported bundlers are:

• browserify
• webpack

Please note: Unfortunately the line numbers in stack traces have an offset of +2 (browserify) / +1 (webpack). This is caused by generated code that is added during the bundling process. I'm working on that ... :)

###browserify

var b = browserify(),
    bundleSrc;

// Add rewire as browserify middleware
// @see https://github.com/substack/node-browserify/blob/master/doc/methods.markdown#busefn
b.use(require("rewire").bundlers.browserify);

b.addEntry("entry.js");
bundleSrc = b.bundle();

###webpack

var webpackOptions = {
    output: "bundle.js"
};

// This function modifies the webpack options object.
// It adds a postLoader and postProcessor to the bundling process.
// @see https://github.com/webpack/webpack#programmatically-usage
require("rewire").bundlers.webpack(webpackOptions);

webpack("entry.js", webpackOptions, function () {});

Changelog

1.0.1

• Fixed crash when a global module has been used in the browser