Flatten an array

flattenArray.js

//flattens an array of arbitrarily nested arrays without the use of Array.prototype.reduce 
//i.e. [1,[2,[3,4]]] => [1,2,3,4]
//returns -1 if input is not an array
function flatten(arr) {
	if(!Array.isArray(arr)) {
  	return -1;
  }
  
  let flat = [];
  for(let i = 0; i < arr.length; i++) {
    let curr = arr[i];
    if(Array.isArray(arr[i])) {
      curr = flatten(curr);
    }
    flat = flat.concat(curr);
  }
  return flat;
}