mdnExample.js

/* Type the word "exit" in any point of your page... */

(function () {
  const sSecret = /* Choose your hidden word...: */ "exit";
  let nOffset = 0;

  document.onkeypress = function(oPEvt) {
    let oEvent = oPEvt || window.event,
        nChr = oEvent.charCode,
        sNodeType = oEvent.target.nodeName.toUpperCase();

    if (nChr === 0 ||
        oEvent.target.contentEditable.toUpperCase() === "TRUE" ||
        sNodeType === "TEXTAREA" ||
        sNodeType === "INPUT" && oEvent.target.type.toUpperCase() === "TEXT") {
      return true;
    }

    if (nChr !== sSecret.charCodeAt(nOffset)) {
      nOffset = nChr === sSecret.charCodeAt(0) ? 1 : 0;
    } else if (nOffset < sSecret.length - 1) {
      nOffset++;
    } else {
      nOffset = 0;
      /* Do something here... */
      alert("Yesss!!!");
      location.assign("https://developer.mozilla.org/");
    }

    return true;
  };
})();