expect 
Notice
This package has been donated to Jest. This means that all future development of expect v21+ will take place at facebook/jest.
You can use jest-codemods to automatically migrate your old tests using [email protected] to the new jest version of expect (>= 21)
Versions prior to v21 will receive limited support and bugfixes, and any future < v21 releases will be published on an npm tag that is not "latest", to avoid causing problems for v21+ users.
[email protected] documentation
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.isSpyThe UMD build is also available on unpkg:
<script src="https://unpkg.com/expect@%3C21/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()Aliases:
toBeTruthy
toNotExist
expect(object).toNotExist([message])
Asserts the given object is falsy.
expect(null).toNotExist()Aliases:
toBeFalsy
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/)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)Aliases:
toBeAn
toBeA(string)
expect(object).toBeA(string, [message])
expect(object).toBeAn(string, [message])
Asserts the typeof the given object is string.
expect(2).toBeA('number')Aliases:
toBeAn
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)Aliases:
toNotBeAn
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')Aliases:
toNotBeAn
toMatch
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/
}
})toNotMatch
expect(string).toNotMatch(pattern, [message])
expect(object).toNotMatch(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/
}
})toBeLessThan
expect(number).toBeLessThan(value, [message])
expect(number).toBeFewerThan(value, [message])
Asserts the given number is less than value.
expect(2).toBeLessThan(3)Aliases:
toBeFewerThan
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)Aliases:
toBeMoreThan
toBeGreaterThanOrEqualTo
expect(number).toBeGreaterThanOrEqualTo(value, [message])
Asserts the given number is greater than or equal to value.
expect(3).toBeGreaterThanOrEqualTo(2)toInclude
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
toExclude
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:
toNotContaintoNotInclude
toIncludeKey(s)
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)
toExcludeKey(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)
(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([fn], [restore])
Creates a spy function with an (optional) implementation and (optional) restore logic. (In order for your provided implementation to be used, you must call andCallThrough.) For this reason, it's better to use andCall if you don't need custom restore logic.
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 and properties
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 "[email protected]"
}).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.
calls
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:
context
spy.calls[index].context
The this value of the call's execution context.
arguments
spy.calls[index].arguments
An array of the arguments passed to the spy for the particular call.
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]{3,6}$/),
'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" />) - expect-predicate Adds assertions based on arbitrary predicates
- expect-enzyme Augments and extends expect to supercharge your enzyme assertions