Construct Huge arrays made out of parts ^_^

partArray.js

function partArray (...parts) {
  let lengths = parts.map(part => part.length)
  let length = lengths.reduce((total, length) => total + length)
  function* iterator () {
    for (const part of parts)
      for (const value of part) yield value
  }
  function item (index) {
    let partIndex = 0
    for (const part of parts) {
      if (index < part.length) {
        return part[index]
      }
      index -= lengths[partIndex]
      partIndex += 1
    }
  }
  return new Proxy([], {
    get (obj, key) {
      if (key === Symbol.iterator) {
        return iterator
      }
      if (key === 'length') {
        return length
      }
      if (key === 'toString') {
        return function () {
          let result = '[ '
          for (var i = 0; i < length; i ++) {
            if (i !== 0) {
              result += ', '
            }
            result += item(i)
          }
          return result + ' ]'
        }
      }
      if (key === 'push') {
        return function (value) {
          parts.push(value)
          lengths.push(1)
          return length++
        }
      }
      if (key === 'concat') {
        return function (...values) {
          const newParts = parts.concat()
          newParts.push(values)
          return partArray(...newParts)
        }
      }
      // TODO: implement the rest...
      if (typeof key === 'symbol' || isNaN(key) || key >= length || key < 0) {
        return undefined
      }
      return item(key | 0)
    }
  })
}

function multiply (input, count) {
  input = input.concat()
  const parts = []
  for (let i = 0; i<count; i++) {
    parts.push(input)
  }
  return partArray(...parts)
}
const input = ['a', 'b', 'c']
const result = multiply(input, 3)
console.log(result instanceof Array)
console.log(Array.isArray(result))
console.log(result.length)
console.log(result[3])
console.log(result[-1])
input.pop()
input.push('d')
input.push('e')
console.log(result.length)
console.log(result[3])
console.log(result.concat('e').toString())
console.log(result.push('f'))
for (const char of result) {
  console.log(char)
}