🗜️ Employing Cantor-Pairing for efficient compression and decompression, reducing the size of a string by half.

cantor-compress.js

// Try it online:
// https://bit.ly/48ZZbOE

function compress(t) {
  function pair(a, b) {
    return 0.5 * (a + b) * (a + b + 1) + b;
  }
  return new Array(Math.ceil(t.length / 2)).fill(0).map((_, i) => String.fromCharCode(pair(t.charCodeAt(i * 2) + 1, t.charCodeAt(i * 2 + 1) + 1 || 0))).join("");
}

function decompress(t) {
  function unpair(n) {
    var w = Math.floor((Math.sqrt(8 * n + 1) - 1) / 2);
    var t = (w ** 2 + w) / 2;
    return [w - (n - t), n - t];
  }
  return t.split("").map(i => {
    var p = unpair(i.charCodeAt(0));
    return String.fromCharCode(p[0] - 1) + (p[1] == 0 ? "" : String.fromCharCode(p[1] - 1));
  }).join("");
}