asymmetric_matchers.md

Jest Asymmetric Matchers :

expect.anything() : is an asymmetric matcher that matches anything that is not equal to undefined or null and it's used with the toEqual() matcher.

it('Testing anything...', () => {
  // Pass
  expect(sayJest()).toEqual(expect.anything());
  // Fail
  expect(null).toEqual(expect.anything());
});

Note: each expect() call represents a partial test of the unit test. it means that a test will pass, if all the partial tests passed. The previous test will fail.

expect.any() : is an asymmetric matcher that takes a constructor and matches any object which is created via the received constructor.

class Person {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }
}

it('Testing any...', () => {
  expect(new Person('mustafa', 21))
    .toEqual(expect.any(Person));
});

expect.arrayContaining() : is an asymmetric matcher that checks whether a given sub array that holds items which are contained in another array.

it('Testing arrayContaining...', () => {
  expect([1, 2, 4, 5, 4])
    .toEqual(expect.arrayContaining([4, 4]));
});

Note: the previous test is same as we replace [4, 4] by [4, 4, 4, .....] it will pass, because once the item is contained in the array then it passed. (ex: a number 4 is contained two times in the array but, it doesn't mean that we match its occurrences)

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