nested arrays of integers into a flat array of integers

flatten.js

var data = [[1,2,[3]],4];

function flatten(array) {
	var output = [];
  array.forEach(function(value) {
  	if(value instanceof Array) {
    		output = output.concat(flatten(value));
    } else {
      	output.push(value)
    }
  });
  return output;
}
console.log(flatten(data));