expect

expect lets you write better assertions.

When you use expect, you write assertions similarly to how you would say them, e.g. "I expect this value to be equal to 3" or "I expect this array to contain 3". When you write assertions in this way, you don't need to remember the order of actual and expected arguments to functions like assert.equal, which helps you write better tests.

You can think of expect as a more compact alternative to Chai or Sinon.JS, just without the pretty website. ;)

Installation

Using npm:

$ npm install --save expect

Then, use as you would anything else:

// using ES6 modules
import expect, { createSpy, spyOn, isSpy } from 'expect'

// using CommonJS modules
var expect = require('expect')
var createSpy = expect.createSpy
var spyOn = expect.spyOn
var isSpy = expect.isSpy

The UMD build is also available on npmcdn:

<script src="https://npmcdn.com/expect/umd/expect.min.js"></script>

You can find the library on window.expect.

Assertions

toExist

expect(object).toExist([message])

Asserts the given object is truthy.

expect('something truthy').toExist()

toNotExist

expect(object).toNotExist([message])

Asserts the given object is falsy.

expect(null).toNotExist()

toBe

expect(object).toBe(value, [message])

Asserts that object is strictly equal to value using ===.

toNotBe

expect(object).toNotBe(value, [message])

Asserts that object is not strictly equal to value using ===.

toEqual

expect(object).toEqual(value, [message])

Asserts that the given object equals value using is-equal.

toNotEqual

expect(object).toNotEqual(value, [message])

Asserts that the given object is not equal to value using is-equal.

toThrow

expect(block).toThrow([error], [message])

Asserts that the given block throws an error. The error argument may be a constructor (to test using instanceof), or a string/RegExp to test against error.message.

expect(function () {
  throw new Error('boom!')
}).toThrow(/boom/)

withArgs

expect(block).withArgs(...args).toThrow([error], [message])

Asserts that the given block throws an error when called with args. The error argument may be a constructor (to test using instanceof), or a string/RegExp to test against error.message.

expect(function (check) {
  if (check === 'bad')
    throw new Error('boom!')
}).withArgs('bad').toThrow(/boom/)

withContext

expect(block).withContext(context).toThrow([error], [message])

Asserts that the given block throws an error when called in the given context. The error argument may be a constructor (to test using instanceof), or a string/RegExp to test against error.message.

expect(function () {
  if (this.check === 'bad')
    throw new Error('boom!')
}).withContext({ check: 'bad' }).toThrow(/boom/)

toNotThrow

expect(block).toNotThrow([message])

Asserts that the given block does not throw.

toBeA(constructor)

expect(object).toBeA(constructor, [message])
expect(object).toBeAn(constructor, [message])

Asserts the given object is an instanceof constructor.

expect(new User).toBeA(User)
expect(new Asset).toBeAn(Asset)

toBeA(string)

expect(object).toBeA(string, [message])
expect(object).toBeAn(string, [message])

Asserts the typeof the given object is string.

expect(2).toBeA('number')

toNotBeA(constructor)

expect(object).toNotBeA(constructor, [message])
expect(object).toNotBeAn(constructor, [message])

Asserts the given object is not an instanceof constructor.

expect(new Asset).toNotBeA(User)
expect(new User).toNotBeAn(Asset)

toNotBeA(string)

expect(object).toNotBeA(string, [message])
expect(object).toNotBeAn(string, [message])

Asserts the typeof the given object is not string.

expect('a string').toNotBeA('number')
expect(2).toNotBeAn('object')

toMatch

expect(string).toMatch(pattern, [message])

Asserts the given string matches pattern, which must be a RegExp.

expect('a string').toMatch(/string/)

toBeLessThan

expect(number).toBeLessThan(value, [message])
expect(number).toBeFewerThan(value, [message])

Asserts the given number is less than value.

expect(2).toBeLessThan(3)

toBeLessThanOrEqualTo

expect(number).toBeLessThanOrEqualTo(value, [message])

Asserts the given number is less than or equal to value.

expect(2).toBeLessThanOrEqualTo(3)

toBeGreaterThan

expect(number).toBeGreaterThan(value, [message])
expect(number).toBeMoreThan(value, [message])

Asserts the given number is greater than value.

expect(3).toBeGreaterThan(2)

toBeGreaterThanOrEqualTo

expect(number).toBeGreaterThanOrEqualTo(value, [message])

Asserts the given number is greater than or equal to value.

expect(3).toBeGreaterThanOrEqualTo(2)

(array) toInclude

expect(array).toInclude(value, [comparator], [message])
expect(array).toContain(value, [comparator], [message])

Asserts the given array contains value. The comparator function, if given, should compare two objects and either return false or throw if they are not equal. It defaults to assert.deepEqual.

expect([ 1, 2, 3 ]).toInclude(3)

(array) toExclude

expect(array).toExclude(value, [comparator], [message])
expect(array).toNotContain(value, [comparator], [message])

Asserts the given array does not contain value. The comparator function, if given, should compare two objects and either return false or throw if they are not equal. It defaults to assert.deepEqual.

expect([ 1, 2, 3 ]).toExclude(4)

(object) toInclude

expect(object).toInclude(value, [comparator], [message])
expect(object).toContain(value, [comparator], [message])

Asserts the given object contains all keys and values in value, recursively. The comparator function, if given, should compare two objects and either return false or throw if they are not equal. It defaults to assert.deepEqual.

expect({ a: 1, b: 2 }).toInclude({ b: 2 })
expect({ a: 1, b: 2, c: { d: 3 } }).toInclude({ b: 2, c: { d: 3 } })

(string) toInclude

expect(string).toInclude(value, [message])
expect(string).toContain(value, [message])

Asserts the given string contains value.

expect('hello world').toInclude('world')
expect('hello world').toContain('world')

(string) toExclude

expect(string).toExclude(value, [message])
expect(string).toNotContain(value, [message])

Asserts the given string does not contain value.

expect('hello world').toExclude('goodbye')
expect('hello world').toNotContain('goodbye')

(spy) toHaveBeenCalled

expect(spy).toHaveBeenCalled([message])

Asserts the given spy function has been called at least once.

expect(spy).toHaveBeenCalled()

(spy) toNotHaveBeenCalled

expect(spy).toNotHaveBeenCalled([message])

Asserts the given spy function has not been called.

expect(spy).toNotHaveBeenCalled()

(spy) toHaveBeenCalledWith

expect(spy).toHaveBeenCalledWith(...args)

Asserts the given spy function has been called with the expected arguments.

expect(spy).toHaveBeenCalledWith('foo', 'bar')

Chaining Assertions

Every assertion returns an Expectation object, so you can chain assertions together.

expect(3.14)
  .toExist()
  .toBeLessThan(4)
  .toBeGreaterThan(3)

Spies

expect also includes the ability to create spy functions that can track the calls that are made to other functions and make various assertions based on the arguments and context that were used.

var video = {
  play: function () {},
  pause: function () {},
  rewind: function () {}
}

var spy = expect.spyOn(video, 'play')

video.play('some', 'args')

expect(spy.calls.length).toEqual(1)
expect(spy.calls[0].context).toBe(video)
expect(spy.calls[0].arguments).toEqual([ 'some', 'args' ])
expect(spy).toHaveBeenCalled()
expect(spy).toHaveBeenCalledWith('some', 'args')

spy.restore()
expect.restoreSpies()

createSpy

expect.createSpy()

Creates a spy function.

var spy = expect.createSpy()

spyOn

expect.spyOn(target, method)

Replaces the method in target with a spy.

var video = {
  play: function () {}
}

var spy = expect.spyOn(video, 'play')
video.play()

spy.restore()

restoreSpies

expect.restoreSpies()

Restores all spies created with expect.spyOn(). This is the same as calling spy.restore() on all spies created.

// mocha.js example
beforeEach(function () {
  expect.spyOn(profile, 'load')
})

afterEach(function () {
  expect.restoreSpies()
})

it('works', function () {
  profile.load()
  expect(profile.load).toHaveBeenCalled()
})

Spy methods

andCall

spy.andCall(fn)

Makes the spy invoke a function fn when called.

var dice = createSpy().andCall(function () {
  return (Math.random() * 6) | 0
})

andCallThrough

spy.andCallThrough()

Makes the spy call the original function it's spying on.

spyOn(profile, 'load').andCallThrough()

var getEmail = createSpy(function () {
  return "hi@gmail.com"
}).andCallThrough()

andReturn

spy.andReturn(object)

Makes the spy return a value.

var dice = expect.createSpy().andReturn(3)

andThrow

spy.andThrow(error)

Makes the spy throw an error when called.

var failing = expect.createSpy()
  .andThrow(new Error('Not working'))

restore

spy.restore()

Restores a spy originally created with expect.spyOn().

reset

spy.reset()

Clears out all saved calls to the spy.

Extending expect

You can add your own assertions using expect.extend and expect.assert:

expect.extend({
  toBeAColor() {
    expect.assert(
      this.actual.match(/^#[a-fA-F0-9]{6}$/),
      'expected %s to be an HTML color',
      this.actual
    )
    return this
  }
})

expect('#ff00ff').toBeAColor()

Extensions

• expect-element Adds assertions that are useful for DOM elements
• expect-jsx Adds things like expect(ReactComponent).toEqualJSX(<TestComponent prop="yes" />)