Object.assign()

app.js

import lib from './library'
lib({
  foo: false
})

exercise.js

// Exercise: Implement your own version of Object.assign()

// Spec:
// http://exploringjs.com/es6/ch_oop-besides-classes.html#Object_assign

// Hint: Object.keys()

function objectAssign (target, ...overrides) {
  // ... code ...

  // Cheat:
  return Object.assign(target, ...overrides)
}

for (const expectation of [
  // Test: Return target
  () => {
    const target = {}
    const result = objectAssign(target)
    console.log(result === target)
  },

  // Test: Target should be modified
  () => {
    const target = {}
    const override = { foo: 123 }
    const result = objectAssign(target, override)
    console.log(typeof result.foo !== 'undefined')
  },

  // Test: Overrides should not be modified
  () => {
    const target = { foo: 123 }
    const override = { bar: 123 }
    const result = objectAssign(target, override)
    console.log(typeof override.foo === 'undefined')
  },

  // Test: Sequentially copy from first to last override
  () => {
    const target = {}
    const first = { foo: 123 }
    const second = { foo: 456 }
    const result = objectAssign(target, first, second)
    console.log(result.foo === second.foo)
  }
]) expectation()

// Console output:
// true
// true
// true
// true

library.js

const defaults = {
  foo: true,
  bar: 123
}
export default function (overrides) {
  const options = Object.assign({}, defaults, overrides)
  console.log(options) // has foo from override & bar from defaults
  console.log(overrides) // unchanged
  console.log(defaults) // unchanged
}

solution.js

// Using `for ... of`
function objectAssign (target, ...overrides) {
  for (const override of overrides)
    for (const key of Object.keys(override))
      target[key] = override[key]
  return target
}

// Using `forEach`
function objectAssign (target, ...overrides) {
  overrides.forEach(override => {
    Object.keys(override)
      .forEach(key => {
        target[key] = override[key]
      })
  })
  return target
}