ES-6 Gotchas

destructuring-within-while.js

const arrayIterator = (array) => {
  let i = 0;
  
  return () => {
    const done = i < array.length;
    
    return {
      done,
      value: done ? undefined : array[i++]
    }
  }
}

const brokenIteratorSum = (iterator) => {
  let done, value,
      sum = 0;;
  
  while (({done, value} = iterator(), !done)) {
    sum += value
  }
  return sum
}
    
brokenIteratorSum(arrayIterator([1, 4, 9, 16, 25]))
  //=> repl: cannot add a declaration here in node type WhileStatement
  
const iteratorSum = (iterator) => {
  let i,
      sum = 0;;
  
  while ((i = iterator(), !i.done)) {
    sum += i.value;
  }
  return sum
}
    
iteratorSum(arrayIterator([1, 4, 9, 16, 25]))
  //=> 55

This is a journey... Into sound.