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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.
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');
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.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 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 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.
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 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 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 = {
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 "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(ReactComponent).toEqualJSX(<TestComponent prop="yes" />)
FAQs
This package exports the `expect` function used in [Jest](https://jestjs.io/). You can find its documentation [on Jest's website](https://jestjs.io/docs/expect).
The npm package expect receives a total of 24,813,839 weekly downloads. As such, expect popularity was classified as popular.
We found that expect demonstrated a not healthy version release cadence and project activity because the last version was released a year ago. It has 2 open source maintainers collaborating on the project.
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