how to write unit tests with jest code example

Example 1: testing with jest

npm i jest --save -dev
-> In Package.json change the Script object(watchAll - jest will run auto)
"test" : "jest --watchAll"
-> test file should be named such way that re used by jest can recognize it
like filename.<test> or <spec> or skip it.js
Ex: sum.test.js or sum.spec.js or sum.js
-> require the target for testing file in this test file then enjoy testing.
-> visit jestWebsite-> Docs.

Example 2: different types of unit testing in jest

describe('in the math global object', () => {
 
  describe('the random function', () => {
    it('should return a number', () => {
      expect(typeof Math.random()).toEqual('number');
    })
    it('should return a number between 0 and 1', () => {
      const randomNumber = Math.random();
      expect(randomNumber).toBeGreaterThanOrEqual(0);
      expect(randomNumber).toBeLessThan(1);
    })
  });
 
  describe('the round function', () => {
    it('should return a rounded value of 4.5 being 5', () => {
      expect(Math.round(4.5)).toBe(5);
    })
  });
 
})