105 lines
1.7 KiB
Markdown
105 lines
1.7 KiB
Markdown
---
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title: expect.js
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category: JavaScript libraries
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updated: 2017-09-02
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weight: -1
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---
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### Using
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```
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npm install --save-dev expect
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```
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{: .-setup}
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```js
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// using ES6 modules
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import expect, { createSpy, spyOn, isSpy } from 'expect'
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```
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```js
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// using CommonJS modules
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var expect = require('expect')
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var createSpy = expect.createSpy
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var spyOn = expect.spyOn
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var isSpy = expect.isSpy
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```
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Expect is a library for assertions in tests.
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See: [mjackson/expect](https://github.com/mjackson/expect)
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### Assertions
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```js
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expect(x).toBe(y)
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.toBe(val)
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.toEqual(val)
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.toThrow(err)
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.toExist() // aka: toBeTruthy()
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.toNotExist() // aka: toBeFalsy()
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.toBeA(constructor)
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.toBeA('string')
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.toMatch(/expr/)
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.toBeLessThan(n)
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.toBeGreaterThan(n)
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.toBeLessThanOrEqualTo(n)
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.toBeGreaterThanOrEqualTo(n)
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.toInclude(val) // aka: toContain(val)
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.toExclude(val)
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.toIncludeKey(key)
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.toExcludeKey(key)
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```
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Also: `toNotBe`, `toNotEqual`, etc for negatives.
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### Chaining assertions
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```js
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expect(3.14)
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.toExist()
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.toBeLessThan(4)
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.toBeGreaterThan(3)
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```
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Assertions can be chained.
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### Spies
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```js
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const video = {
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play: function () { ··· }
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}
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```
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{: .-setup}
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```js
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spy = expect.spyOn(video, 'play')
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```
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```js
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spy = expect.spyOn(···)
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.andCallThrough() // pass through
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.andCall(fn)
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.andThrow(exception)
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.andReturn(value)
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```
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### Assertions on spies
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```js
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expect(spy.calls.length).toEqual(1)
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expect(spy.calls[0].context).toBe(video)
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expect(spy.calls[0].arguments).toEqual([ 'some', 'args' ])
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expect(spy.getLastCall().arguments).toEqual(...)
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```
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```js
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expect(spy).toHaveBeenCalled()
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expect(spy).toHaveBeenCalledWith('some', 'args')
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```
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### References
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- <https://www.npmjs.com/package/expect>
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- <https://github.com/mjackson/expect>
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