expectjs.md

   1 ---
   2 title: expect.js
   3 category: JavaScript libraries
   4 layout: 2017/sheet
   5 updated: 2017-09-02
   6 weight: -1
   7 ---
   8
   9 ### Using
  10
  11 ```
  12 npm install --save-dev expect
  13 ```
  14 {: .-setup}
  15
  16 ```js
  17 // using ES6 modules
  18 import expect, { createSpy, spyOn, isSpy } from 'expect'
  19 ```
  20
  21 ```js
  22 // using CommonJS modules
  23 var expect = require('expect')
  24 var createSpy = expect.createSpy
  25 var spyOn = expect.spyOn
  26 var isSpy = expect.isSpy
  27 ```
  28
  29 Expect is a library for assertions in tests.
  30 See: [mjackson/expect](https://github.com/mjackson/expect)
  31
  32 ### Assertions
  33
  34 ```js
  35 expect(x).toBe(y)
  36   .toBe(val)
  37   .toEqual(val)
  38   .toThrow(err)
  39   .toExist()          // aka: toBeTruthy()
  40   .toNotExist()       // aka: toBeFalsy()
  41   .toBeA(constructor)
  42   .toBeA('string')
  43   .toMatch(/expr/)
  44   .toBeLessThan(n)
  45   .toBeGreaterThan(n)
  46   .toBeLessThanOrEqualTo(n)
  47   .toBeGreaterThanOrEqualTo(n)
  48   .toInclude(val)     // aka: toContain(val)
  49   .toExclude(val)
  50   .toIncludeKey(key)
  51   .toExcludeKey(key)
  52 ```
  53
  54 Also: `toNotBe`, `toNotEqual`, etc for negatives.
  55
  56 ### Chaining assertions
  57
  58 ```js
  59 expect(3.14)
  60   .toExist()
  61   .toBeLessThan(4)
  62   .toBeGreaterThan(3)
  63 ```
  64
  65 Assertions can be chained.
  66
  67 ### Spies
  68
  69 ```js
  70 const video = {
  71   play: function () { ··· }
  72 }
  73 ```
  74 {: .-setup}
  75
  76 ```js
  77 spy = expect.spyOn(video, 'play')
  78 ```
  79
  80 ```js
  81 spy = expect.spyOn(···)
  82   .andCallThrough()      // pass through
  83   .andCall(fn)
  84   .andThrow(exception)
  85   .andReturn(value)
  86 ```
  87
  88 ### Assertions on spies
  89
  90 ```js
  91 expect(spy.calls.length).toEqual(1)
  92 expect(spy.calls[0].context).toBe(video)
  93 expect(spy.calls[0].arguments).toEqual([ 'some', 'args' ])
  94 expect(spy.getLastCall().arguments).toEqual(...)
  95 ```
  96
  97 ```js
  98 expect(spy).toHaveBeenCalled()
  99 expect(spy).toHaveBeenCalledWith('some', 'args')
 100 ```
 101
 102 ### References
 103
 104 - <https://www.npmjs.com/package/expect>
 105 - <https://github.com/mjackson/expect>