Code from Unit testing in JavaScript Part 3 - Test runners

order-total.js

function orderTotal(order) {
  return order.items.reduce((prev, cur) => cur.price * (cur.quantity || 1) + prev, 0)
}

module.exports = orderTotal

order-total.test.js

const orderTotal = require('./order-total')

it('Quantity', () =>
  expect(orderTotal({
    items: [
      { 'name': 'Dragon candy', price: 2, quantity: 3 }
    ]
  })).toBe(6))

it('No quantity specified', () =>
  expect(orderTotal({
    items: [
      { 'name': 'Dragon candy', price: 3 }
    ]
  })).toBe(3)
)

it('Happy path (Example 1)', () =>
  expect(orderTotal({
    items: [
      { name: 'Dragon food', price: 8, quantity: 1 },
      { name: 'Dragon cage (small)',  price: 800, quantity: 1 }
    ]
  })).toBe(808)
)
it('Happy path (Example 2)', () =>
  expect(orderTotal({
    items: [
      { name: 'Dragon collar', price: 20, quantity: 1 },
      { name: 'Dragon chew toy',  price: 40, quantity: 1 }
    ]
  })).toBe(60))

package.json

{
  "name": "test-runners",
  "version": "1.0.0",
  "description": "",
  "main": "code-and-tests.js",
  "scripts": {
    "test": "jest",
    "watch": "jest --watch *.js"
  },
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "jest": "^21.2.1"
  }
}