Kaboom save and load buttons (https://pwmarcz.pl/kaboom/)

saveload.user.js

// ==UserScript==
// @name        Kaboom save and load
// @namespace   Kaboom
// @match       https://pwmarcz.pl/kaboom/
// @grant       none
// @version     1.0
// @author      umnikos
// ==/UserScript==

function isObject(item) {
  return (item && typeof item === 'object' && !Array.isArray(item));
}

// recursing version of Object.assign
function mergeDeep(target, source) {
  if (isObject(target) && isObject(source)) {
    for (const key in source) {
      if (isObject(source[key])) {
        if (!target[key]) Object.assign(target, { [key]: {} });
        mergeDeep(target[key], source[key]);
      } else {
        Object.assign(target, { [key]: source[key] });
      }
    }
  }
  return target;
}

function saveGame() {
  return JSON.stringify(game);
}

function loadGame(mygame) {
  const width = parseInt(document.getElementById('width').value, 10);
  const height = parseInt(document.getElementById('height').value, 10);
  const numMines = parseInt(document.getElementById('numMines').value, 10);

  const gameElement = document.getElementById('game');
  gameElement.innerHTML = '';
  game = new Game(width, height, numMines);
  game = mergeDeep(new Game(), JSON.parse(mygame));
  game.mount(gameElement);
  updateSettings();
  updateSize();
}

function exportGame() {
  console.log("exporting game...");
  saved_game = saveGame();
  let filename = "kaboom_save.txt";
  let data = saved_game;
  let type = "text/plain";
  
  var file = new Blob([data], {type: type});
  var output = document.createElement("a");
  var url = URL.createObjectURL(file);
  output.href = url;
  output.download = filename;
  output.click();
}

function importGame() {
  var input = document.createElement('input');
  input.type = 'file';
  input.onchange = (function(e) {
    var file = e.target.files[0]; 
    var reader = new FileReader();
    reader.readAsText(file,'UTF-8');
    reader.onload = (function(readerEvent) {
      var content = readerEvent.target.result;
      saved_game = content;
      loadGame(saved_game);
    });
  });
  input.click();
}

function makeSaveLoadButtons() {
  let undoButton = document.getElementById('undo');
  let mydiv = document.createElement('div');
  undoButton.parentElement.parentElement.insertBefore(mydiv, undoButton.parentElement);
  
  saveButton = document.createElement('button');
  saveButton.onclick = (function() {
    saved_game = saveGame();
  });
  saveButton.classList = undoButton.classList;
  saveButton.innerHTML = "Save";
  
  
  loadButton = document.createElement('button');
  loadButton.onclick = (function() {
    loadGame(saved_game);
  });
  loadButton.classList = undoButton.classList;
  loadButton.innerHTML = "Load";
  
  exportButton = document.createElement('button');
  exportButton.onclick = (function() {
    exportGame();
  });
  exportButton.classList = undoButton.classList;
  exportButton.innerHTML = "Save & Export";
  
  importButton = document.createElement('button');
  importButton.onclick = (function() {
    importGame();
  });
  importButton.classList = undoButton.classList;
  importButton.innerHTML = "Import & Load";
  
  // order to taste
  mydiv.append(importButton);
  mydiv.append(" ");
  mydiv.append(loadButton);
  mydiv.append(" ");
  mydiv.append(saveButton);
  mydiv.append(" ");
  mydiv.append(exportButton);
}

(function() {
  window.mergeDeep = mergeDeep;
  window.saveGame = saveGame;
  window.loadGame = loadGame;
  makeSaveLoadButtons();
  saved_game = null;
})();