Jest Unit testing — Use of Matchers (Part1)

What is matcher in Jest?

Jest uses matchers to test the unit test cases in different ways like matching the equal values, truthiness, numbers, strings, and so on.

Basic matchers

//strict equality
expect(10).toBe(10)
//strict equality (!==)
expect(10).not.toBe(5)
//Deep equality
expect([6, 9]).toEqual([6, 9])
expect({ a: 7, b: undefined}).toEqual({ a: 7})
//strict equality (version Jest 23+)
expect({ a: undefined, b: 9}).not.toStrictEqual({ b: 10}) 

Numbers

expect(9).toBeGreaterThan(5)
expect(5).toBeGreaterThanOrEqual(5)
expect(4).toBeLessThan(2)
expect(3).toBeLessThanOrEqual(3)
expect(0.3 + 0.2).toBeCloseTo(0.5, 5)
expect(NaN).toEqual(expect.any(Number))

Strings

expect('hello jest').toMatch('jest')
expect('string').toEqual(expect.any(String))
expect('toffee').toMatch(/ff/)
expect('pizza').not.toMatch('burger')
expect(['pizza', 'toffee']).toEqual([expect.stringContaining('zz'), expect.stringMatching(/ff/)])

Truthiness

// Matches anything that an if statement treats as true 
(not false, 0, '', null, undefined, NaN)
xeTruthy()
// Matches anything that an if statement treats as false 
(false, 0, '', null, undefined, NaN)
expect('').toBeFalsy()
// Matches only null
expect(null).toBeNull()
// Matches only undefined
expect(undefined).toBeUndefined()
// The opposite of toBeUndefined
expect(7).toBeDefined()
// Matches true or false
expect(true).toEqual(expect.any(Boolean))

Arrays

expect([]).toEqual(expect.any(Array))
expect(['Apple', 'Banana', 'Orange']).toHaveLength(3)
expect(['Apple', 'Apple', 'Apple']).toContain('Apple')
expect([{ a: 10 }, { a: 20 }]).toContainEqual({ a: 10 })
expect(['Apple', 'Banana', 'Orange']).toEqual(expect.arrayContaining(['Apple', 'Orange']))

Objects

expect({ a: 1 }).toHaveProperty('a')
expect({ a: 1 }).toHaveProperty('a', 1)
expect({ a: { b: 1 } }).toHaveProperty('a.b')
expect({ a: 1, b: 2 }).toMatchObject({ a: 1 })
expect({ a: 1, b: 2 }).toMatchObject({
  a: expect.any(Number),
  b: expect.any(Number)
})
expect([{ a: 1 }, { b: 2 }]).toEqual([
  expect.objectContaining({ a: expect.any(Number) }),
  expect.anything()
])

so these are the basic matchers to be used while writing unit test cases. For more reference refer below link

Using Matchers · Jest

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