Jest test 101

jest-test.jest.js

// @ package.json
{
  "scripts": {
    "jest": "NODE_ENV=test DB_USER=ivan-wang ./node_modules/jest/bin/jest.js",
    "jest-watch": "NODE_ENV=test DB_USER=ivan-wang ./node_modules/jest/bin/jest.js --watch --onlyChanged"
  },
  "jest": {
    "testEnvironment": "node",
    "automock": true,
    "rootDir": "",
    "unmockedModulePathPatterns": [
      "<rootDir>/node_modules/lodash/",
      "<rootDir>/node_modules/firebase/"
    ]
  },
  "devDependencies": {
    "babel-jest": "^16.0.0",
    "jest": "^16.0.1"
  }
}

// @ test.js
jest.unmock('./dummy')

import doSomething from './dummy'
import { plusOne } from './module'

describe('Do something', () => {
  plusOne.mockImplementation(num => num + 1)

  it('returns 6', () => {
    expect(plusOne(0)).toEqual(1)
    expect(doSomething()).toEqual(6)
  })
})

// @ dummy.js
import { plusOne } from './module'

export default function doSomething() {
  const num = 5
  return plusOne(5)
}

// @ module.js
export function plusOne (num) {
  return num + 1
}