TIL; JavaScript Jest - matchers

matchers

• matchers allow you to test values by comparing the actual value to the expected value
• expect returns an expectation object. you call matchers on this object to achieve the comparison of actual to expected.
• toBe matcher uses [Object.is] to test exact equality.
• if you want to check the value of an object, use toEqual or toStrictEqual instead.
  • what does ‘check the value of an object’ mean?
• toBe vs toEqual vs toStrictEqual. when to use which?
  • prefer toStrictEqual over toEqual, cos toEqual simply ignores undefined values
  • under which conditions would you want to ignore undefined values?
• you can easily test the opposite of a matcher using .not. eg: expect(a+b).not.toStrictEqual(0)

truthiness

• matchers that allow you to be specific about the kind of truthy value you want
• there’s 5 of them
• toBeNull matches only null
• toBeUndefined matches only undefined
• toBeDefined is the opposite of toBeUndefined
• toBeTruthy matches anything that an if statement treats as true
• toBeFalsy matches anything that an if statement treats as false
• you should use the matcher that most precisely corresponds to what you want your code to be doing

numbers

• there’s 7 matchers that can be used for numbers
• toBeGreaterThan
• toBeGreatherThanOrEqual
• toBeLessThan
• toBeLessThanOrEqual
• toBe
• toEqual

• toBeCloseTo ; to be used for floating point equality. cos we don’t want a test to depend on a tiny rounding error.
  

  const value = 0.1 + 0.2;
  expect(value).toBe(0.3); // this won't work cos of rounding error
  expect(value).toBeCloseTo(0.3); // this works
  

• toBe and toEqual are equivalent for numbers

strings

• you can check strings against regular expressions with toMatch
  • expect('Christoph').toMatch(/stop/)

arrays and iterables

• you can check if an array or iterable contains a particular item using toContain
  • expect(['diapers', 'paper towels']).toContain('paper towels');
• JS iterables include the ff
  • string
  • array
  • TypedArray
  • Map
  • Set
  • Segments (returned by Intl, Segmenter, prototype, segment())

an iterable is defined by the fact that its prototype object implements an @@iterator method

exceptions

• to test whether a particular function throws an error when it’s called, use toThrow
• to check the error message, you can pass a regex to the toThrow() matcher
• the function that throws an exception needs to be invoked within a wrapping function otherwise the toThrow() assertion will fail.