nasty buggy javascript exponent thing

myexpt.js

function myexpt(x, y) {
  var expt = 1;
  if (x < 0) {
    // XXX - need to check Number.isFinite(parseFloat(y)) and cast for modulus via parseInt
    if (y % 2 != 0) {
      expt = -1 * myexpt(Math.abs(x), y);
    } else {
      expt = myexpt(Math.abs(x), y);
    }
  } else {
    if (y < 0) {
      expt = 1 / myexpt(x, Math.abs(y));
    } else if (y == 0) {
      expt = 1;
    } else if (y > 0) {
      expt = x * myexpt(x, y - 1);
    }
  }
  return(expt);
}