Codility Solution Task 2: Decimal Zip

decimalZip = (A, B) => {
    if ( typeof A !== "number" || typeof B !== "number" ) {
    return undefined;
  }
  var a = JSON.stringify(A).split('')
  var b = JSON.stringify(B).split('')
  var c = [];
  var longer = a.length > b.length ? a : b;
  for (var i = 0; i < longer.length; i++) {
    if (a.length) {
      c.push(a.shift())
    } else {
      c.push(b.splice(b[i]))
    }
    if (b.length) {
      c.push(b.shift())
    } else {
      c.push(a.splice(a[i]))
    }
    
  }
  if (a.length !== b.length) {
    var rest = a.length > b.length ? a : b;
    c.push(rest.join(''))
  }
  return parseInt(c.join(''));
}