the `extend` function in underscore

extend.js

// in the namespace of `_`
// The simplest extend
function simpleExtend(obj, src) {
  for (let key in src) {
    obj[key] = src[key]
  }
  return obj
}

// the two different type of configuration of key assigners

_.keys = function(obj) {
  if (Object.keys) return Object.keys(obj)
  
  const keys = []
  for (let key in obj) {
    if (obj.hasOwnProperty(key))  keys.push(key)
  }
  return keys
}

_.allKeys = function(obj) {
  const keys = []
  for (let key in obj) {
    keys.push(key)
  }
  return keys
}

// -------------------------------------

function createAssigners(keyAssigner, undefinedOnly = false) {
  return function(obj, ...args) {
    if (!args.length) return obj
    
    args.forEach(target => {
      const keys = keyAssigner(target)
      for (let key of keys) {
        if (!undefinedOnly || !obj[key]) {
          obj[key] = target[key]
        }
      }
    })
    
    return obj
  }
}

_.extend = createAssigners(_.keys)
_.extendAll = createAssigners(_.allKeys)
_.defaultExtendAll = createAssigners(_.allKeys, true)