expect

What is expect?

The 'expect' npm package is a library for writing test assertions. It is commonly used in conjunction with testing frameworks like Jest or Mocha to validate the behavior of JavaScript code. It provides a range of assertion types and matchers that allow developers to write expressive and readable tests.

What are expect's main functionalities?

Basic Assertions

This feature allows you to assert that a value matches exactly what you expect. The 'toBe' matcher compares with ===.

expect(2 + 2).toBe(4);

Object Property Assertions

With this feature, you can assert that an object has a specific property with a certain value. The 'toHaveProperty' matcher checks for the existence and value of a property in an object.

expect({ name: 'Alice', age: 30 }).toHaveProperty('name', 'Alice');

Exception Testing

This feature is used to test if a function throws an exception when it is executed. The 'toThrow' matcher is used to assert that an error is thrown with a specific message.

expect(() => { throw new Error('failure'); }).toThrow('failure');

Array Containment

This feature allows you to assert that an array contains a specific item. The 'toContain' matcher checks if an array includes the expected item.

expect(['Alice', 'Bob', 'Eve']).toContain('Bob');

Asynchronous Assertions

This feature enables you to write assertions for asynchronous code. The 'resolves' matcher waits for a promise to resolve and then checks the resolved value.

expect(Promise.resolve('success')).resolves.toBe('success');

Other packages similar to expect

chai

Chai is a BDD/TDD assertion library for node and the browser that can be paired with any javascript testing framework. It offers a similar range of matchers and assertions as expect, and it provides a fluent chainable language to construct assertions.

should

Should.js is an expressive, readable, framework-agnostic assertion library. The key difference is in the style of assertions it offers, using a more fluent chainable API that reads like English.

assert

Assert is a module that provides a set of assertion functions for verifying invariants. It is built into Node.js and is less feature-rich compared to expect, but it serves as a simple way to write assertions in a Node.js environment.

jest

Jest is a delightful JavaScript Testing Framework with a focus on simplicity. It comes with its own assertion library, which provides functionality similar to expect, as expect is actually part of Jest.

README

expect [][build]

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.

Installation

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.

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 deep-equal.

toNotEqual

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

Asserts that the given object is not equal to value using deep-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 User).toBeA(User)
expect(new Asset).toBeAn(Asset)

toNotBeA(string)

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

Asserts the typeof the given object is not string.

expect(2).toBeA('number')

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)

toBeGreaterThan

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

Asserts the given number is greater than value.

expect(3).toBeGreaterThan(2)

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)

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)

(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')

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 () {}
}

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 "[e-mail]"
}).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().

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
    )
  }
})

expect('#ff00ff').toBeAColor()

Extensions

• expect-jsx Add things like expect(ReactComponent).toEqualJSX(<TestComponent prop="yes" />)

Issues

Please file issues on the issue tracker on GitHub.

[build]: https://travis-ci.org/mjackson/expect)