Char wise, byte foolish.

payloadPack.js

const pack = s =>
  s.match(/^[\u0000-\u00ff]*$/)
    ? s
      .split("")
      .map(s => s.charCodeAt())
      .reduce(
        (pairs, c) =>
          (
            !c || pairs[pairs.length - 1].length === 2
              ? pairs.push(...(c? [[c]] : [[c], []]))
              : pairs[pairs.length - 1].push(c)
          )
          && pairs,
        [[]]
      )
      .filter(pair => pair.length)
      .map(pair => String.fromCharCode(
        pair.length === 2? (pair[0] << 8) + pair[1] : pair[0]
      ))
      .join("")
    : null;

const unpack = s => unescape(escape(s).replace(/u(..)/g, "$1%"));


const exampleStr = "This is a packed ascii string.";

console.log(exampleStr, exampleStr.length); // This is a packed ascii string. – 30
console.log(pack(exampleStr), pack(exampleStr).length); // 周楳⁩猠愠灡捫敤⁡獣楩⁳瑲楮朮 – 15
console.log(exampleStr === unpack(pack(exampleStr))); // true

// eval('console.log("Hello!")')
eval(unescape(escape("捯湳潬攮汯木≈敬汯™)").replace(/u(..)/g, "$1%")));