what is jest

                                    beforeAll(() => console.log('1 - beforeAll'));
afterAll(() => console.log('1 - afterAll'));
beforeEach(() => console.log('1 - beforeEach'));
afterEach(() => console.log('1 - afterEach'));
test('', () => console.log('1 - test'));
describe('Scoped / Nested block', () => {
  beforeAll(() => console.log('2 - beforeAll'));
  afterAll(() => console.log('2 - afterAll'));
  beforeEach(() => console.log('2 - beforeEach'));
  afterEach(() => console.log('2 - afterEach'));
  test('', () => console.log('2 - test'));
});

// 1 - beforeAll
// 1 - beforeEach
// 1 - test
// 1 - afterEach
// 2 - beforeAll
// 1 - beforeEach
// 2 - beforeEach
// 2 - test
// 2 - afterEach
// 1 - afterEach
// 2 - afterAll
// 1 - afterAll
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                                    function sum(a, b) {
  return a + b;
}
module.exports = sum;
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                                    const sum = require('./sum');

test('adds 1 + 2 to equal 3', () => {
  expect(sum(1, 2)).toBe(3);
});
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                                    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.