The simplest Jest example.

package.json

{
  "name": "foo",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "jest"
  },
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "jest": "^16.0.0"
  }
}

sum.js

module.exports = (a, b) => a + b

sum.spec.js

const sum = require('./sum')

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