Give a simple example of Jest test case

Let's write a test for a function that adds two numbers in sum.js file:

const sum = (a, b) => a + b;

export default sum;

Create a file named sum.test.js which contains actual test:

import sum from './sum';

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

And then add the following section to your package.json:

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

Finally, run yarn test or npm test and Jest will print a result:

$ yarn test
PASS ./sum.test.js
✓ adds 1 + 2 to equal 3 (2ms)