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
throw
s 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
throw
s 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
throw
s 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 User).toBeA(User)
expect(new Asset).toBeAn(Asset)
expect(object).toNotBeA(string, [message])
expect(object).toNotBeAn(string, [message])
Asserts the typeof
the given object
is not string
.
expect(2).toBeA('number')
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).toBeGreaterThan(value, [message])
expect(number).toBeMoreThan(value, [message])
Asserts the given number
is greater than value
.
expect(3).toBeGreaterThan(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')
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 () {}
}
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()
.
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-jsx Add things like
expect(ReactComponent).toEqualJSX(<TestComponent prop="yes" />)
Please file issues on the issue tracker on GitHub.