Jest Testing Framework – Setup

Jest is an open-source testing framework that has become increasingly popular recently. Developed by Facebook and built into the popular packagecreate-react-app. Jest comes with built-in mocking and assertion abilities. In addition, Jest runs your tests concurrently in parallel, providing a smoother, faster test run.

Setting up a new project with Jest

Using npm:  npm i -D jest

Using yarn:yarn add --dev jest

Simple Jest Test:

In package.json

{
  "scripts": {
    "test": "jest"
  }
}

In functions.js file

const functions = {
  add: (num1, num2) => num1 + num2
};

module.exports=functions;

In functions.test.js file

const functions=require('./functions');

describe("Addition", () => {
  it("should add two numbers ", () => {
    expect(functions.add(2,2,)).toBe(4);
  });
});

Run yarn test or npm test, see the results below

 PASS  ./functions.test.js
  Addition
    ✓ should add two numbers  (11ms)

Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        2.035s
Ran all test suites.

Failure scenario:

const functions=require('./functions');

describe("Addition", () => {
  it("should add two numbers ", () => {
    expect(functions.add(2,2,)).toBe(5);   #here expecting 5 instead of 4.
  });
});

 FAIL  ./functions.test.js
  Addition
    ✕ should add two numbers  (9ms)
  ● Addition › should add two numbers 
    expect(received).toBe(expected) // Object.is equality
    Expected: 5
    Received: 4

      4 | describe("Addition", () => {
      5 |   it("should add two numbers ", () => {
    > 6 |     expect(functions.add(2,2,)).toBe(5); 
        |                                 ^
      7 |   });
      8 | });
      9 | 

      at Object.toBe (functions.test.js:6:33)

Test Suites: 1 failed, 1 total
Tests:       1 failed, 1 total
Snapshots:   0 total
Time:        3.692s
Ran all test suites.
npm ERR! Test failed.  See above for more details.