Mocking function dependencies

When running functions locally you sometimes want to change the behavior of modules which connect to external services. For example you might be accessing a database, API or external system that you don't want to, or can't access from your local environment.

The Cloud Functions Emulator provides a way to inject mock versions of Node.js modules imported into your function.

1. Enable the use of mocks:

    functions config set useMocks true
    functions restart
   

2. Edit mocks.js and mock the dependencies you want

    // You can create handcrafted mocks, or use a mocking library
    var sinon = require('sinon');
   
    /**
     * Called when the require() method is called
     * @param {String} func The name of the current function being invoked
     * @param {String} module The name of the module being required
     */
    exports.onRequire = function (func, module) {
        // Return an object or module to override the named module argument
        if (module === 'redis') {
            // Create a mock of redis
            var mockRedis = sinon.mock(require('redis'));
            mockRedis.expects('createClient').returns({});
   
            // Mock more methods as needed...
   
            return mockRedis;
        }
        return undefined;
    };
   

3. You don't need to change your function code at all!

    exports.helloRedis = function (event, callback) {
      var redis = require('redis'); // this will be a mock
      cvar lient = redis.createClient();
   
      // ...
    };