Determine if a given number is in range without using comparison branches and operators

number-in-range.js

/**
 * This was intended for checking that an ASCII character is in range.
 * But this turns out to be a function that can be used for checking if a number is in range too.
 * 
 * Assuming, `MIN` and `MAX` are hardcoded to the code point of the character 'a' and
 * the character 'z', respectively, the following equation will always return positive integers
 * in the difference of the `MIN` and the `MAX`:
 *
 *     (MAX - x) and (x - MIN) must always be positive for any of the results.
 *
 * When any of which produces a negative value, we know that the given number denoted by `x` is
 * out of range.
 */
const MIN = 65; // 'a'
const MAX = 90; // 'z'

function inRange(a) {
  const aInt = a >> 0; /** Turns Infinity, NaN into 0, which makes no changes to valid numbers */
  const lowerBound = aInt - MIN;
  const upperBound = MAX - aInt;
  const lowerBoundSign = lowerBound >>> 31; /** Get the sign bit of the lower bound result */
  const upperBoundSign = upperBound >>> 31; /** Get the sign bit of the upper bound result */

  // Expected output (left) which technically is an inverse of an OR truth table (right)
  // |       | R |       |       | OR |
  // | 0 | 0 | 1 |       | 0 | 0 |  0 | 
  // | 0 | 1 | 0 |       | 0 | 1 |  1 |
  // | 1 | 0 | 0 |       | 1 | 0 |  1 |
  // | 1 | 1 | 0 |       | 1 | 1 |  1 |
  return !(lowerBoundSign | upperBoundSign);
};

[-65, -1, 0, 64, 65, 70, 74, 80, 85, 90, 91, Infinity, NaN].filter(n => inRange(n));