function isDigitStart(code) {
  return (code >= 48 && code <= 57) /* 0..9 */
    || code === 0x2B /* + */
    || code === 0x2D /* - */
    || code === 0x2E; /* . */
}
Checks if the character is or belongs to a number.
[0-9]|+|-|.

@param {number} code the character to check
@returns {boolean} check result