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. You just have to add rewire as a middleware (see below).

Installation

npm install rewire

For older node versions:
rewire is tested with node 0.6.x - 0.8.x. I recommend to run the unit tests via mocha in the rewire-folder before using rewire with other node versions.

Use with browserify:

var b = require("browserify")({debug: true});

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

After that rewire works exactly as in node.

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
// cache just call rewire.reset().
// Do this after every single 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.