Iterative flatten indefinite nested array

flatten.js

function flatten(input) {
  if (!input) return
  var output = [];
  while(input.length)
    if(input[0] instanceof Array)
      input.unshift.apply(input, input.shift())
    else
      output.push(input.shift())
  return output
}

Unshifting is very expensive. It is better to operate at the end of the array (rather to modify it at beginning).

@lifeamit depends on the language and how the array is built internally - most are contiguous, so adding onto the end often requires copying everything over to a new place in memory, anyway. This is versus a linked list where you can obviously just link to data anywhere else in memory without any issue. Please illuminate me if I'm missing something!