mockery/examples/example1.js

Simplifying the use of mocks with Node.js

/*
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*/

/*
 * This example demonstrates the use of Mockery to allow a mock to be used even
 * when its requiring module has already been loaded and cached by Node. We do
 * not use a test framework in this example, so that we control the sequence of
 * calls to our "tests". This lets us show that tests using the original module
 * dependencies can run before and / or after tests using our mock, without any
 * interference in either direction.
 *
 * Notes:
 * 1. If you change useCleanCache to 'false' in the mock test, that test will
 *    fail, because the module to be mocked was already cached by the previous
 *    test.
 * 2. If, in addition to the above, you remove the call to the real test before
 *    the call to the mock test, the mock test will succeed but the second real
 *    test will fail, because the mock is now cached by Node.
 * 3. If, in addition to both of the above, you amend the registration of the
 *    module under test to ask that it be unhooked, the remaining tests will
 *    pass, because Node will reload that module for the third test.
 * 4. The 'useCleanCache' option is useful in particular because it avoids the
 *    complexities inherent in the above combinations, and lets your tests run
 *    in any order without the risk of cross-test interference.
 */

"use strict";

var assert = require('assert'),
    mockery = require('../mockery'),
    moduleUnderTest = '../test/fixtures/intermediary';

// This is the mock we will use to replace the functionality of 'fake_module',
// which is used by 'intermediary'.
var mock_fake_module = {
    foo: function () {
        return 'mocked foo';
    }
};

/*
 * Here we test 'intermediary' in its original state, without using any mocks.
 * Thus the original 'fake_module' will be loaded and used by 'intermediary'.
 */
function testWithRealFoo() {
    var intermediary = require(moduleUnderTest),
        result = intermediary.bar();
    console.log("testWithRealFoo: " + result);
    assert.equal(result, "real foo");
}

/*
 * Here we test 'intermediary' while mocking its dependency, 'fake_module'. Our
 * mock provides different results, so that we can verify the test.
 *
 * Note that when a test framework is being used, it would be typical for (some
 * of) the Mockery calls to be placed in setUp() and tearDown() functions.
 */
function testWithMockFoo() {
    // Register the module under test (i.e. 'intermediary') as an allowable
    // module, so that mockery won't complain about it being loaded.
    mockery.registerAllowable(moduleUnderTest);
    // Register our mock for 'fake_module', using the path that 'intermediary'
    // will use to 'require' it. This tells mockery to provide our mock any
    // time this path is passed to 'require'.
    mockery.registerMock('./fake_module', mock_fake_module);
    // Enable mockery and tell it to use a clean cache. By using a clean cache,
    // Node will reload 'intermediary', causing it in turn to re-require its
    // 'fake_module' dependency, at which point mockery will provide our mock.
    // Without the clean cache, 'intermediary' will not be reloaded, because
    // Node knows it is already loaded (and cached). This means that mockery
    // has no opportunity to provide the mock, since 'fake_module' was already
    // loaded by 'intermediary' and will not be re-required.
    mockery.enable({ useCleanCache: true });

    var intermediary = require(moduleUnderTest),
        result = intermediary.bar();
    console.log("testWithMockFoo: " + result);
    assert.equal(result, "mocked foo");

    // Now that we're done with our test, we need to disable mockery so that it
    // doesn't continue intercepting 'require' calls from, for example, a test
    // framework.
    mockery.disable();
    // Finally, we clean up by deregistering the mock and allowable that we
    // registered at the beginning of the test.
    mockery.deregisterAll();
}

// Mainline code
//
// We run a test using the original modules first, which causes Node to load
// and cache all of the modules. Then we run a test using a mock, showing that
// it is provided even though the original was cached. Finally, we re-run the
// test using the original modules, showing that everything was unhooked after
// the previous test.
testWithRealFoo();
testWithMockFoo();
testWithRealFoo();