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. ;)

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.

expect(object).toExist([message])

Asserts the given object is truthy.

expect('something truthy').toExist()

Aliases:

• toBeTruthy

expect(object).toNotExist([message])

Asserts the given object is falsy.

expect(null).toNotExist()

Aliases:

• toBeFalsy

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).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)

Aliases:

• toBeAn

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

Asserts the typeof the given object is string.

expect(2).toBeA('number')

Aliases:

• toBeAn

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)

Aliases:

• toNotBeAn

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')

Aliases:

• toNotBeAn

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

Asserts the given string or object matches a pattern. When using a string, pattern must be a RegExp. When using an object, pattern may be anything acceptable to tmatch.

expect('a string').toMatch(/string/)
expect({
  statusCode: 200,
  headers: {
    server: 'nginx/1.6.5'
  }
}).toMatch({
  headers: {
    server: /nginx/
  }
})

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

Asserts the given string or object does not match a pattern. When using a string, pattern must be a RegExp. When using an object, pattern may be anything acceptable to tmatch.

expect('a string').toMatch(/string/)
expect({
  statusCode: 200,
  headers: {
    server: 'nginx/1.6.5'
  }
}).toNotMatch({
  headers: {
    server: /apache/
  }
})

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

Asserts the given number is less than value.

expect(2).toBeLessThan(3)

Aliases:

• toBeFewerThan

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)

Aliases:

• toBeMoreThan

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

Asserts that a given value is included (or "contained") within another. The actual value may be an array, object, or a string. The comparator function, if given, should compare two objects and return false if they are not equal. The default is to use isEqual.

expect([ 1, 2, 3 ]).toInclude(3)
expect({ a: 1, b: 2 }).toInclude({ b: 2 })
expect({ a: 1, b: 2, c: { d: 3 } }).toInclude({ b: 2, c: { d: 3 } })
expect('hello world').toInclude('world')

Aliases:

• toContain

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

Asserts that a given value is not included (or "contained") within another. The actual value may be an array, object, or a string. The comparator function, if given, should compare two objects and return false if they are not equal. The default is to use isEqual.

expect([ 1, 2, 3 ]).toExclude(4)
expect({ a: 1, b: 2 }).toExclude({ c: 2 })
expect({ a: 1, b: 2 }).toExclude({ b: 3 })
expect({ a: 1, b: 2, c: { d: 3 } }).toExclude({ c: { d: 4 } })
expect('hello world').toExclude('goodbye')

Aliases:

• toNotContain
• toNotInclude

expect(object).toIncludeKeys(keys, [comparator], [message])
expect(object).toIncludeKey(key, [comparator], [message])

Asserts that the given object (may be an array, or a function, or anything with keys) contains all of the provided keys. The optional parameter comparator is a function which if given an object and a string key, it should return a boolean detailing whether or not the key exists in the object. By default, a shallow check with Object.prototype.hasOwnProperty is performed.

expect({ a: 1 }).toIncludeKey('a')
expect({ a: 1, b: 2 }).toIncludeKeys([ 'a', 'b' ])

Aliases:

• toContainKey(s)

expect(object).toExcludeKeys(keys, [comparator], [message])
expect(object).toExcludeKey(key, [comparator], [message])

Asserts that the given object (may be an array, or a function, or anything with keys) does not contain any of the provided keys. The optional parameter comparator is a function which if given an object and a string key, it should return a boolean detailing whether or not the key exists in the object. By default, a shallow check with Object.prototype.hasOwnProperty is performed.

expect({ a: 1 }).toExcludeKey('b')
expect({ a: 1, b: 2 }).toExcludeKeys([ 'c', 'd' ])

Aliases:

• toNotContainKey(s)
• toNotIncludeKey(s)

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 = {
  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()

expect.createSpy()

Creates a spy function.

var spy = expect.createSpy()

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()

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 "[email protected]"
}).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.

spy.calls

An array of objects representing all saved calls to the spy.

You can use the length of the calls array to make assertions about how many times you expect the spy to have been called.

expect(spy.calls.length).toEqual(3)

You can also use the array to make assertions about each individual call. Each call object contains the following properties:

spy.calls[index].context

The this value of the call's execution context.

spy.calls[index].arguments

An array of the arguments passed to the spy for the particular call.

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()

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