expect

Write better assertions

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.

Using npm:

$ npm install expect

Then with a module bundler like webpack, use as you would anything else:

// using an ES6 transpiler, like babel 
import expect, { createSpy, spyOn, isSpy } from 'expect'
 
// not using an ES6 transpiler 
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.

expect(object).toExist([message])

Asserts the given object is truthy.

expect('something truthy').toExist()

expect(object).toNotExist([message])

Asserts the given object is falsy.

expect(null).toNotExist()

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

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

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

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

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

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

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

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

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/)

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/)

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/)

expect(block).toNotThrow([message])

Asserts that the given block does not throw.

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)

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

Asserts the typeof the given object is string.

expect(2).toBeA('number')

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)

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')

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

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

expect('a string').toMatch(/string/)

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

Asserts the given number is less than value.

expect(2).toBeLessThan(3)

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

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

expect(2).toBeLessThanOrEqualTo(3)

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

Asserts the given number is greater than value.

expect(3).toBeGreaterThan(2)

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

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

expect(3).toBeGreaterThanOrEqualTo(2)

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)

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)

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')

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')

expect(spy).toHaveBeenCalled([message])

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

expect(spy).toHaveBeenCalled()

expect(spy).toNotHaveBeenCalled([message])

Asserts the given spy function has not been called.

expect(spy).toNotHaveBeenCalled()

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

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

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

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

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

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 = {
  playfunction () {},
  pausefunction () {},
  rewindfunction () {}
}
 
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()

expect.createSpy()

Creates a spy function.

var spy = expect.createSpy()

expect.spyOn(target, method)

Replaces the method in target with a spy.

var video = {
  playfunction () {}
}
 
var spy = expect.spyOn(video, 'play')
video.play()
 
spy.restore()

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.andCall(fn)

Makes the spy invoke a function fn when called.

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

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()

spy.andReturn(object)

Makes the spy return a value.

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

spy.andThrow(error)

Makes the spy throw an error when called.

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

spy.restore()

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

spy.reset()

Clears out all saved calls to the spy.

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
    )
  }
})
 
expect('#ff00ff').toBeAColor()
  • expect-element Adds assertions that are useful for DOM elements
  • expect-jsx Adds things like expect(ReactComponent).toEqualJSX(<TestComponent prop="yes" />)

Please file issues on the issue tracker on GitHub.