Flatten an integer array in JavaScript

flattenArray.js

function flattenIntegerArray(array) {
	var ret = [];
  for (var i = 0; i < array.length; i++) {
  	if (Array.isArray(array[i])) {
    	//Add items from the sub array into the returned array
    	Array.prototype.push.apply(ret, flattenIntegerArray(array[i]));
    } else if (Number.isInteger(array[i])) {
    	ret.push(array[i]);
    } else {
    	// In JavaScript, you can't guarantee that the type
      // is going to be consistent
    	throw new Error('Invalid type');
    }
  }
  return ret;
}


var arr = [[1,2,[3]],4];
console.log(JSON.stringify(flattenIntegerArray(arr))); // [1,2,3,4]

var arr = [1, [3,5,2,[6,8,[4,[33,7,[9]]]]]];
console.log(JSON.stringify(flattenIntegerArray(arr))); // [1,3,5,2,6,8,4,33,7,9]