jsmock

jsmock

Mocking framework for javascript, inspired by googlemock C++ framework. This project is still under construction ...

• Installation
• User Guide
  • Creating Mocks
  • Defining Expectation
  • Specifying Matcher
  • Specifying Cardinality
  • Adding Actions
  • Verifying Mocks
  • Cleaning Mocks
• Examples

Installation

jsmock is published on npm

npm install --save-dev jsmock

User Guide

All examples provided below assume using mocha/chai test framework, although jsmock can be used with any framework of your choice.

Creating Mocks

const Mock = require('jsmock').Mock;

let foo = {
  bar: (a, b) => {
    return a + b
  }
};

let fooMock = new Mock(foo);

Now fooMock is a mock object wrapping foo. All functions of original object have been replaced and any call to foo.bar will cause an UnexpectedCall error to be thrown.

expect(foo.bar.bind(foo)).to.throw(Error);

Defining Expectation

Expectation of function call is defined by calling expectCall on mock object.

fooMock.expectCall('bar');

By default this will setup an expectation of single call to foo.bar function with any arguments. As there is no action specified yet, nothing will be returned and no side effects will be observed.

Specifying Matcher

Matcher validates that call of mocked function is valid for given expectation. If no explicit matcher is specified expectation will be executed for any call to mocked function. Matcher can be specified as a predicate or simply as an arguments list to be verified against actual call.

fooMock.expectCall('bar', (a,b) => a > b);
foo.bar(3,2); // OK - 3 > 2
foo.bar(1,4); // KO

fooMock.expectCall('bar', 1, 8);
foo.bar(1, 8); // OK
foo.bar(1, 0); // KO 

Matcher can be specified directly in arguments of the expectCall method or by calling matching function on mock object. Note that expectCall returns mock object making call chain possible:

fooMock.expectCall('bar').matching((a,b) => a < b);
fooMock.expectCall('bar').matching(1,4);

Specifying Cardinality

Cardinality specifies number of expected calls to given function. jsmock provides two ways of specifying expectation cardinality. It can be provided explicitly through one of expectation methods, or it can be calculated automatically from list of specified actions. If cardinality is specified explicitly it takes precedence over one calculated from action list.

fooMock.expectCall('bar').times(2); // Expect bar to be called twice
fooMock.expectCall('bar').atLeast(1); // Expect bar to be called at least one time
fooMock.expectCall('bar').atMost(4); // Expect bar to be called 1 - 4 times
fooMock.expectCall('bar').between(3,5); // Expect bar to be called 3 - 5 times

// Note that 
fooMock.expectCall('bar').atMost(MaxCallCount);
// is equivalent of 
fooMock.expectCall('bar').between(1, MaxCallCount);

Cardinality can be specified only once for given expectation.

Adding Actions

Action is an object encapsulating function to be executed instead of the original code on mocked object. Each expectation can have multiple actions defined with specific cardinality. Actions are executed in the order of creation.

fooMock.expectCall('bar')
  .willOnce((a,b) => a * b) // First call will return multiplication of arguments
  .willTwice((a,b) => a + b) // Second and third will return sum of arguments
  .willRepeatedly((a,b) => b); // All following calls will return second argument

If action specifying method is feed with function it will use it as a callback for actual mocked function execution. If parameter of any other type is provided it will be returned to the caller at execution time.

fooMock.expectCall('bar')
  .willOnce(4) // Return 4 on first call
  .willTwice(7) // return 7 on next 2 calls
  .willRepeatedly(0); // All following calls will return 0

The willRepeatedly method specifies action with unlimited number of potential calls, thus any other attempt to add more actions to the expectation will cause error. Also note that willRepeatedly doesn't return expectation object so it isn't suitable for chaining.

In js and nodejs it's very common to provide callback as the last argument in the function call. Often the only purpose of the mock is to execute that callback with some predefined arguments. This kind of action can be created easily using will...Invoke versions of already presented functions:

fsMock.expectCall('readdir')
  .willOnceInvoke(null, ['a.js', 'b.js']);

// Which is equivalent of
fsMock.expectCall('readdir')
  .willOnceInvoke((path, cb) => cb(null, ['a.js', 'b.js']));

Actions and Cardinality

Combination of cardinality and action specifiers can build virtually any expectation.

fooMock.expectCall('bar')
  .times(5) // Total number of calls expected to be 5
  .willOnce(3) // First call returns 3
  .willRepeatedly(0); // Next 4 calls returns 0

fooMock.expectCall('bar')
  .atLeast(8)
  .willRepeatedly(1); // Will always return 1

Verifying Mocks

Mock object will yield errors directly in case of unexpected calls or violation of cardinality upper bound (more calls than expected). Verification of cardinality lower bound has to be done explicitly by the user, at the end of the test.

let foo = new Foo();
let fooMock = new Mock(foo);
fooMock.expectCall('bar')
  .times(2) // Lower bound of the cardinality is 2
  .willRepeatedly(6);

expect(foo.bar()).to.be.equal(6); // We make only one call to bar 

fooMock.verify(); // Will throw Error 

Call to verify methods cleans up all previously setup expectations.

Cleaning Mocks

Creation of the mock over an existing object modifies its functions. To restore object to its original state you need to explicitly call cleanup method.

const fs = require('fs');
const Mock = require('jsmock').Mock;

let fsMock = new Mock(fs);

// Setup some expectations
fsMock.expectCall('readdir')
  .willOnce((path, cb) => cb(null, ['index.html']));

testedObject.doSomeStuff();

fsMock.verify();

// Once you don't need to use fs in your tests
fsMock.cleanup();

Examples

Some examples of potential "real life" usage can be found in example.js test file.

it('Should perform some fs action', (done) => {
  let fsMock = new Mock(fs);
  fsMock.expectCall('readdir')
    .matching(path => path === '/tmp')
    .willOnce((path, cb) => cb(null, ['a.js', 'b.js']));

  foo.readTemp((err, files) => {
    expect(err).to.be.null;
    expect(files).to.deep.equal(['a.js', 'b.js']);
    fooMock.verify(done);
  });
});