FlattenArrayIterative.js

function FlattenArray(arr){
  var flattened = [];
  
  while(arr.length){
    var element = arr.pop(); //start from end
    
    //if (element instanceof Array)
    //if (Object.prototype.toString.call(element) === '[object Array]')
    if (Array.isArray(element)){
      arr = arr.concat(element);
    }
    else{
      flattened.push(element);
    }
  }
  
  return flattened.reverse();
}

FlattenArrayRecursive.js

function FlattenArray(arr){
  var flattened = [];
  
  for (var i = 0; i < arr.length; i++){
    var element = arr[i];
    
    if (Array.isArray(element)){
      flattened = flattened.concat(FlattenArray(element));
    }
    else{
      flattened.push(element);
    }
  }
  
  return flattened;
}

FlattenArrayRecursiveES6.js

function FlattenArray(arr){
  var reducer = (flatten, original) => flatten.concat((Array.isArray(original) ? FlattenArray(original) : original));
  
  return arr.reduce(reducer, []);
}