polymorphic map() over Google Apps Script iterators and Arrays.

gistfile1.js

function _map( coll, f ){
  var res = []
  var isIterator = ( typeof coll.next == "function" )
  var isArray = ( coll instanceof Array )
  var hasFunc = ( typeof f == "function" )
  var append = function(e){ res.push( hasFunc ? f(e) : e ) }
  if( isIterator ){
    while( coll.hasNext() ){ append( coll.next() ) }
  } else if ( isArray ) {
    coll.forEach(append)
  } else {
    throw new Error("unknown collection type " + coll )
  }
  return res
}


// mapping a Google Drive iterator
_map( DriveApp.getFolderById(id).getFiles(), function(file){ return file.getName() } )

// mapping over an array
_map( [1, 2, 3], function(n){ return n * 2 })