jsmock

jsmock

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

• Installation
• User Guide
  • Creating Mocks
  • Defining Expectations
    • General Syntax
    • 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.

Mock provides an API to define and verify expectations on all function calls performed on reference to original object. A standard unit test involving mocks will consist of following steps:

1. Mock external dependencies of the test subject
2. Define expectations on created mocks
3. Execute actual tests on the subject
4. Verify mock objects
5. Cleanup

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 Expectations

Expectation is a main component of a mock oriented test. It describes what kind of interactions with mocked object are expected during test execution. In addition Expectation also defines the actions that should be taken to mock the original object behavior.

Expectation for given function is defined by 3 objects:

• Matcher
• Cardinality
• Action List

Matcher decides if current function call is valid for the expectation. Usually it means that function was called with expected argument list. Cardinality describes how many times the expected call should occur. Consequently Action List provides actions to be performed by mocked object for each call.

General Syntax

Expectation is created by calling expectCall on mock object. Then it can be set up with dedicated methods:

fooMock.expectCall('bar')
  .matching(1, 4)
  .times(4)
  .willOnce((a, b) => a + b)
  .willRepeatedly((a, b) => a - b);

Above code will create expectation on call to foo.bar(1,4) that should happen exactly 4 times. On the first call sum of the arguments will be returned, following 3 call with return their difference.

Specifying Matcher

Matcher is an object checking 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.

In jsmock matchers come in 3 flavours:

• Predicate Matcher - applies call arguments to provided predicate
• Strict Argument Matcher - call argument list must match exactly matcher argument list
• Weak Argument Matcher - first N call arguments must match matcher argument list

Matcher can be specified during Expectation creation with expectCall function, or later with one of available Expectation methods.

Method	Description
Mock.expectCall(fn, ...args)	Creates an expectation with Strict Argument Matcher from ...args
Expectation.matching(...args)	Creates a Strict Argument Matcher from ...args
Expectation.with(...args)	Alias of Expectation.matching
Expectation.matchingAtLeast(...args)	Creates a Weak Argument Matcher from ...args
Expectation.withAtLest(...args)	Alias of Expectation.matchingAtLeast

Predicate Matcher is created automatically if one of above methods is called with only one argument that happens to be a function.

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

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

fooMock.expectCall('bar').with(5,6); // Strict Argument Matcher
foo.bar(5,6); // OK
foo.bar(5,1); // KO 

fooMock.expectCall('bar').matchingAtLeast(3); // Weak Argument Matcher
foo.bar(3,6); // OK
foo.bar(3,9); // OK 
foo.bar(1,3); // KO 

Argument Type Matchers

jsmock comes with a family of predefined argument type matchers, that can be helpful if we care more about the type of argument provided to the call than its actual value.

Matcher	Description
Matcher.ANY	Checks only presence of an argument in a call, doesn't care about actual type
Matcher.OBJECT	Checks if given argument is an object
Matcher.NUMBER	Checks if given argument is a number
Matcher.STRING	Checks if given argument is a string
Matcher.BOOLEAN	Checks if given argument is a boolean
Matcher.FUNCTION	Checks if given argument is a function
Matcher.ARRAY	Checks if given argument is an array

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

fooMock.expectCall('bar').with(Matcher.ANY, Matcher.NUMBER);
foo.bar(1,2); // OK
foo.bar('a',2); // OK
foo.bar([1,2,3], true); // KO

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.

Method	Description
Expectation.times(N)	Matching call should occur exactly N times
Expectation.atLeast(N)	Matching call should occur at least N times
Expectation.atMost(N)	Matching call should occur at least once and at most N times
Expectation.between(M,N)	Matching call should occur at least M times and at most N times

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 mocked object code. 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 nodejs (and js in general) 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 action create methods

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

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

Method	Description
Expectation.willOnce(k)	Adds an action to be executed once
Expectation.willTwice(k)	Adds an action to be executed twice
Expectation.willRepeatedly(k)	Adds an action to be executed until expectation cardinality is fulfilled
Expectation.willOnceInvoke(...args)	Adds an invoker action to be executed once
Expectation.willTwiceInvoke(...args)	Adds an invoker action to be executed twice
Expectation.willRepeatedlyInvoke(...args)	Adds an invoker action to be executed until expectation cardinality is fulfilled

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.