Home

Awesome

rewire

Easy monkey-patching for node.js unit tests

Coverage Status

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

Please note: The current version of rewire is only compatible with CommonJS modules. See Limitations.

<br>

Installation

npm install rewire

<br />

Introduction

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. With just 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 multiple 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"]
    }
});

__set__ returns a function which reverts the changes introduced by this particular __set__ call

var revert = myModule.__set__("port", 3000);

// port is now 3000
revert();
// port is now the previous value

For your convenience you can also use the __with__ method which reverts the given changes after it finished.

myModule.__with__({
    port: 3000
})(function () {
    // within this function port is 3000
});
// now port is the previous value again

The __with__ method is also aware of promises. If a thenable is returned all changes stay until the promise has either been resolved or rejected.

myModule.__with__({
    port: 3000
})(function () {
    return new Promise(...);
}).then(function () {
    // now port is the previous value again
});
// port is still 3000 here because the promise hasn't been resolved yet
<br />

Limitations

Babel's ES module emulation<br> During the transpilation step from ESM to CJS modules, Babel renames internal variables. Rewire will not work in these cases (see #62). Other Babel transforms, however, should be fine. Another solution might be switching to babel-plugin-rewire.

Variables inside functions<br> Variables inside functions can not be changed by rewire. This is constrained by the language.

// myModule.js
(function () {
    // Can't be changed by rewire
    var someVariable;
})()

Modules that export primitives<br> rewire is not able to attach the __set__- and __get__-method if your module is just exporting a primitive. Rewiring does not work in this case.

// Will throw an error if it's loaded with rewire()
module.exports = 2;

Globals with invalid variable names<br> rewire imports global variables into the local scope by prepending a list of var declarations:

var someGlobalVar = global.someGlobalVar;

If someGlobalVar is not a valid variable name, rewire just ignores it. In this case you're not able to override the global variable locally.

Special globals<br> Please be aware that you can't rewire eval() or the global object itself.

<br />

API

rewire(filename: String): rewiredModule

Returns a rewired version of the module found at filename. Use rewire() exactly like require().

rewiredModule.__set__(name: String, value: *): Function

Sets the internal variable name to the given value. Returns a function which can be called to revert the change.

rewiredModule.__set__(obj: Object): Function

Takes all enumerable keys of obj as variable names and sets the values respectively. Returns a function which can be called to revert the change.

rewiredModule.__get__(name: String): *

Returns the private variable with the given name.

rewiredModule.__with__(obj: Object): Function<callback: Function>

Returns a function which - when being called - sets obj, executes the given callback and reverts obj. If callback returns a promise, obj is only reverted after the promise has been resolved or rejected. For your convenience the returned function passes the received promise through.

<br />

Caveats

Difference to require()<br> 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.

Globals are imported into the module's scope at the time of rewiring<br> Since rewire imports all gobals into the module's scope at the time of rewiring, property changes on the global object after that are not recognized anymore. This is a problem when using sinon's fake timers after you've called rewire().

Dot notation<br> Although it is possible to use dot notation when calling __set__, it is strongly discouraged in most cases. For instance, writing myModule.__set__("console.log", fn) is effectively the same as just writing console.log = fn. It would be better to write:

myModule.__set__("console", {
    log: function () {}
});

This replaces console just inside myModule. That is, because rewire is using eval() to turn the key expression into an assignment. Hence, calling myModule.__set__("console.log", fn) modifies the log function on the global console object.

<br />

webpack

See rewire-webpack

<br />

License

MIT