Flat Array

flat.array.js

function parseElem(elem){
 
 if(typeof elem == 'number'){
   return [elem];
 }
 var result = [];
 for(var count =0; count < elem.length; count++){
   result = result.concat(parseElem(elem[count]));
 }
 return result;

}

var input =  [[1,2,[4,7]],5,8,[5,[4,7,[21,34],67],23],9];

var output = parseElem(input);

console.log(output);