rewire

Easy dependency injection for node.js unit testing.

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 or globals like process
• 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).

Good news to all caffeine-addicts: rewire works also with Coffee-Script. Note that in this case CoffeeScript needs to be listed in your devDependencies.

If you want to use rewire also on the client-side take a look at client-side bundlers

Installation

npm install rewire

Examples

Imagine you want to test this module:

// lib/myModule.js

// With rewire you can change all these variables
var fs = require("fs"),
    path = "/somewhere/on/the/disk";

function readSomethingFromFileSystem(cb) {
    console.log("Reading from file system ...");
    fs.readFile(path, "utf8", cb);
}

exports.readSomethingFromFileSystem = readSomethingFromFileSystem;

Now within your test module:

// test/myModule.test.js

var rewire = require("rewire");

var myModule = rewire("../lib/myModule.js");

rewire acts exactly like require. Just with one difference: Your module will now export a special setter and getter for private variables.

myModule.__set__("path", "/dev/null");
myModule.__get__("path"); // = '/dev/null'

This allows you to mock everything in the top-level scope of the module, like the fs-module for example. Just pass the variable name as first parameter and your mock as second.

var fsMock = {
    readFile: function (path, encoding, cb) {
        expect(path).to.equal("/somewhere/on/the/disk");
        cb(null, "Success!");
    }
};
myModule.__set__("fs", fsMock);

myModule.readSomethingFromFileSystem(function (err, data) {
    console.log(data); // = Success!
});

You can also set different variables with one call.

myModule.__set__({
    fs: fsMock,
    path: "/dev/null"
});

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"]
    }
});

Caveats

Difference to require()
Every call of rewire() executes the module again and returns a fresh instance.

rewire("./myModule.js") === rewire("./myModule.js"); // = false

This can especially be a problem if the module is not idempotent like mongoose models.

Changing globals
Be careful, if you do something like this you'll change your global console instance.

myModule.__set__("console.log", function () { /* be quiet */ });

##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

###webpack See rewire-webpack

###browserify If you're using browserify and want to use rewire with browserify please let me know.

##License

MIT