array flatten

flatten.es6

const flat = (array, res) => {
  	// Short circuit the result if empty
	if (array.length < 1 || !(array instanceof Array)) return res;
  	
  	// Set start item
  	let s = array[0];
  
  	// Set remaining items
  	let r = array.slice(1);
  
  	if (s instanceof Array) {
      // This value is an array so lets merge it with the rest of items
      s = s.concat(r);
      return flat(s, res);
    }

  	res.push(s); // Push our item onto the result
  	return flat(r, res); // call flat again on our remaining items and pass result
}