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.
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");
var myModule = rewire("../lib/myModule.js");
myModule.__set__("myPrivateVar", 123);
myModule.__get__("myPrivateVar");
myModule.__set__("fs", {
readFile: function (path, encoding, cb) {
cb(null, "Success!");
}
});
myModule.readSomethingFromFileSystem(function (err, data) {
console.log(data);
});
myModule === require("./myModule.js");
myModule.__set__({
fs: fsMock,
http: httpMock,
someOtherVar: "hello"
});
myModule.__set__({
console: {
log: function () { }
},
process: {
argv: ["testArg1", "testArg2"]
}
});
myModule.__set__("console.log", function () { });
assert.ok(myModule.__get__("currentState") === "idle");
rewire("./myModule.js") === require("./myModule.js");
rewire("./myModule.js", false) === require("./myModule.js");
rewire("./myModule.js") === rewire("./myModule.js");
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)
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.