proxyquire

What is proxyquire?

Proxyquire is a powerful tool for stubbing and mocking dependencies in Node.js unit tests. It allows you to override dependencies during testing without modifying the actual code, making it easier to isolate and test individual modules.

What are proxyquire's main functionalities?

Basic Dependency Override

This feature allows you to override a dependency with a mock implementation. In this example, the './dependency' module's 'myFunction' is replaced with a mock function that returns 'mocked value'.

const proxyquire = require('proxyquire');
const myModule = proxyquire('./myModule', {
  './dependency': { myFunction: () => 'mocked value' }
});
console.log(myModule.useDependency()); // Outputs: 'mocked value'

Override Multiple Dependencies

This feature allows you to override multiple dependencies at once. In this example, both './dependency1' and './dependency2' are replaced with mock implementations.

const proxyquire = require('proxyquire');
const myModule = proxyquire('./myModule', {
  './dependency1': { myFunction1: () => 'mocked value 1' },
  './dependency2': { myFunction2: () => 'mocked value 2' }
});
console.log(myModule.useDependencies()); // Outputs: 'mocked value 1 and mocked value 2'

Conditional Overrides

This feature allows you to conditionally override dependencies based on certain conditions, such as environment variables. In this example, the './dependency' module is only mocked if the TEST_ENV environment variable is set.

const proxyquire = require('proxyquire');
const myModule = proxyquire('./myModule', {
  './dependency': process.env.TEST_ENV ? { myFunction: () => 'mocked value' } : require('./dependency')
});
console.log(myModule.useDependency()); // Outputs: 'mocked value' if TEST_ENV is set, otherwise the real value

Other packages similar to proxyquire

sinon

Sinon is a popular library for creating spies, stubs, and mocks in JavaScript. Unlike Proxyquire, which focuses on overriding dependencies, Sinon provides a more comprehensive set of tools for creating test doubles and verifying interactions.

rewire

Rewire is another library that allows you to modify the behavior of modules during testing. It provides a similar functionality to Proxyquire but with a different API. Rewire allows you to access private variables and functions within a module, which can be useful for more fine-grained control.

mock-require

Mock-require is a simpler alternative to Proxyquire that allows you to mock Node.js modules. It provides a straightforward API for replacing modules with mock implementations, but it lacks some of the advanced features of Proxyquire.

README

proxyquire

Proxies nodejs's require in order to make overriding dependencies during testing easy while staying totally unobstrusive.

If you want to stub dependencies for your client side modules, try proxyquireify, a proxyquire for browserify v2.

Features

• no changes to your code are necessary
• non overriden methods of a module behave like the original
• mocking framework agnostic, if it can stub a function then it works with proxyquire
• "use strict" compliant

Example

foo.js:

var path = require('path');

module.exports.extnameAllCaps = function (file) {
  return path.extname(file).toUpperCase();
};

module.exports.basenameAllCaps = function (file) {
  return path.basename(file).toUpperCase();
};

foo.test.js:

var proxyquire =  require('proxyquire')
  , assert     =  require('assert')
  , pathStub   =  { };

// when no overrides are specified, path.extname behaves normally
var foo = proxyquire('./foo', { 'path': pathStub });
assert.equal(foo.extnameAllCaps('file.txt'), '.TXT');

// override path.extname
pathStub.extname = function (file) { return 'Exterminate, exterminate the ' + file; };

// path.extname now behaves as we told it to
assert.equal(foo.extnameAllCaps('file.txt'), 'EXTERMINATE, EXTERMINATE THE FILE.TXT');

// path.basename and all other path module methods still function as before
assert.equal(foo.basenameAllCaps('/a/b/file.txt'), 'FILE.TXT');

Table of Contents generated with DocToc

• Usage
• API
  • Preventing call thru to original dependency
    • Prevent call thru for all future stubs resolved by a proxyquire instance
    • Re-enable call thru for all future stubs resolved by a proxyquire instance
      • All together, now
  • Forcing proxyquire to reload modules
  • Globally override require
    • Caveat
    • Globally override require during module initialization
    • Why is proxyquire messing with my require cache?
    • Globally override require during module runtime
• Backwards Compatibility for proxyquire v0.3.x
• Examples
• More Examples

Usage

Two simple steps to override require in your tests:

• add var proxyquire = require('proxyquire'); to top level of your test file
• proxyquire(...) the module you want to test and pass along stubs for modules you want to override

API

proxyquire({string} request, {Object} stubs)

• request: path to the module to be tested e.g., ../lib/foo
• stubs: key/value pairs of the form { modulePath: stub, ... }
  • module paths are relative to the tested module not the test file
  • therefore specify it exactly as in the require statement inside the tested file
  • values themselves are key/value pairs of functions/properties and the appropriate override

Preventing call thru to original dependency

By default proxyquire calls the function defined on the original dependency whenever it is not found on the stub.

If you prefer a more strict behavior you can prevent callThru on a per module or contextual basis.

If callThru is disabled, you can stub out modules that don't even exist on the machine that your tests are running on. While I wouldn't recommend this in general, I have seen cases where it is legitimately useful (e.g., when requiring global environment configs in json format that may not be available on all machines).

Prevent call thru on path stub:

var foo = proxyquire('./foo', {
  path: {
      extname: function (file) { ... }
    , '@noCallThru': true
  }
});

Prevent call thru for all future stubs resolved by a proxyquire instance

// all stubs resolved by proxyquireStrict will not call through by default
var proxyquireStrict = require('proxyquire').noCallThru();

// all stubs resolved by proxyquireNonStrict will call through by default
var proxyquireNonStrict = require('proxyquire');

Re-enable call thru for all future stubs resolved by a proxyquire instance

proxyquire.callThru();

Call thru config per module wins:

var foo = proxyquire
    .noCallThru()
    .load('./foo', {

        // no calls to original './bar' methods will be made
        './bar' : { toAtm: function (val) { ... } }

        // for 'path' module they will be made
      , path: {
          extname: function (file) { ... }
        , '@noCallThru': false
        }
    });

All together, now

var proxyquire = require('proxyquire').noCallThru();

// all methods for foo's dependencies will have to be stubbed out since proxyquire will not call through
var foo = proxyquire('./foo', stubs);

proxyquire.callThru();

// only some methods for foo's dependencies will have to be stubbed out here since proxyquire will now call through
var foo2 = proxyquire('./foo', stubs);

Forcing proxyquire to reload modules

In most situations it is fine to have proxyquire behave exactly like nodejs require, i.e. modules that are loaded once get pulled from the cache the next time.

For some tests however you need to ensure that the module gets loaded fresh everytime, i.e. if that causes initializing some dependency or some module state.

For this purpose proxyquire exposes the noPreserveCache function.

// ensure we don't get any module from the cache, but to load it fresh every time
var proxyquire = require('proxyquire').noPreserveCache();

var foo1 = proxyquire('./foo', stubs);
var foo2 = proxyquire('./foo', stubs);
var foo3 = require('./foo');

// foo1, foo2 and foo3 are different instances of the same module
assert.notEqual(foo1, foo2);
assert.notEqual(foo1, foo3);

require.preserveCache allows you to restore the behavior to match nodejs's require again.

proxyquire.preserveCache();

var foo1 = proxyquire('./foo', stubs);
var foo2 = proxyquire('./foo', stubs);
var foo3 = require('./foo');

// foo1, foo2 and foo3 are the same instance
ssert.equal(foo1, foo2);
ssert.equal(foo1, foo3);

Globally override require

Use the @global property to override every require of a module, even transitively.

Caveat

You should think very hard about alternatives before using this feature. Why, because it's intrusive and as you'll see if you read on it changes the default behavior of module initialization which means that code runs differently during testing than it does normally.

Additionally it makes it harder to reason about how your tests work.

Yeah, we are mocking fs three levels down in bar, so that's why we have to set it up when testing foo

WAAAT???

If you write proper unit tests you should never have a need for this. So here are some techniques to consider:

• test each module in isolation
• make sure your modules are small enough and do only one thing
• stub out dependencies directly instead of stubbing something inside your dependencies
• if you are testing bar and bar calls foo.read and foo.read calls fs.readFile proceed as follows
  • do not stub out fs.readFile globally
  • instead stub out foo so you can control what foo.read returns without ever even hitting fs

OK, made it past the warnings and still feel like you need this? Read on then but you are on your own now, this is as far as I'll go ;)

Watch out for more warnings below.

Globally override require during module initialization

// foo.js
var bar = require('bar');

module.exports = function() {
  bar();
}

// bar.js
var baz = require('baz');

module.exports = function() {
  baz.method();
}

// baz.js
module.exports = {
  method: function() {
    console.info('hello');
  }
}

// test.js
var stubs = {
  'baz': {
    method: function(val) {
      console.info('goodbye');
    },
    '@global': true
  }
};

var proxyquire = require('proxyquire');

var foo = proxyquire('foo', stubs);
foo();  // 'goodbye' is printed to stdout

Be aware that when using global overrides any module initialization code will be re-executed for each require.

This is not normally the case since node.js caches the return value of require, however to make global overrides work , proxyquire bypasses the module cache. This may cause unexpected behaviour if a module's initialization causes side effects.

As an example consider this module which opens a file during its initialization:

var fs = require('fs')
  , C = require('C');

// will get executed twice
var file = fs.openSync('/tmp/foo.txt', 'w');

module.exports = function() {
  return new C(file);
};

The file at /tmp/foo.txt could be created and/or truncated more than once.

Why is proxyquire messing with my require cache?

Say you have a module, C, that you wish to stub. You require module A which contains require('B'). Module B in turn contains require('C'). If module B has already been required elsewhere then when module A receives the cached version of module B and proxyquire would have no opportunity to inject the stub for C.

Therefore when using the @global flag, proxyquire will bypass the require cache.

Globally override require during module runtime

Say you have a module that looks like this:

module.exports = function() {
  var d = require('d');
  d.method();
};

The invocation of require('d') will happen at runtime and not when the containing module is requested via require. If you want to globally override d above, use the @runtimeGlobal property:

var stubs = {
  'd': {
    method: function(val) {
      console.info('hello world');
    },
    '@runtimeGlobal': true
  }
};

This will cause module setup code to be re-excuted just like @global, but with the difference that it will happen every time the module is requested via require at runtime as no module will ever be cached.

This can cause subtle bugs so if you can guarantee that your modules will not vary their require behaviour at runtime, use @global instead.

Backwards Compatibility for proxyquire v0.3.x

Compatibility mode with proxyquire v0.3.x has been removed.

You should update your code to use the newer API but if you can't, pin the version of proxyquire in your package.json file to ~0.6 in order to continue using the older style.

Examples

We are testing foo which depends on bar:

// bar.js module
module.exports = {
    toAtm: function (val) { return  0.986923267 * val; }
};

// foo.js module
// requires bar which we will stub out in tests
var bar = require('./bar');
[ ... ]

Tests:

// foo-test.js module which is one folder below foo.js (e.g., in ./tests/)

/*
 *   Option a) Resolve and override in one step:
 */
var foo = proxyquire('../foo', {
  './bar': { toAtm: function (val) { return 0; /* wonder what happens now */ } }
});

// [ .. run some tests .. ]

/*
 *   Option b) Resolve with empty stub and add overrides later
 */
var barStub = { };

var foo =  proxyquire('../foo', { './bar': barStub });

// Add override
barStub.toAtm = function (val) { return 0; /* wonder what happens now */ };

[ .. run some tests .. ]

// Change override
barStub.toAtm = function (val) { return -1 * val; /* or now */ };

[ .. run some tests .. ]

// Resolve foo and override multiple of its dependencies in one step - oh my!
var foo = proxyquire('./foo', {
    './bar' : {
      toAtm: function (val) { return 0; /* wonder what happens now */ }
    }
  , path    : {
      extname: function (file) { return 'exterminate the name of ' + file; }
    }
});

More Examples

For more examples look inside the examples folder or look through the tests

Specific Examples:

• test async APIs synchronously: examples/async.
• using proxyquire with Sinon.JS: examples/sinon.