schedule.js

const schedule = (() => {
  const taskQueue = [];
  const runTask = () => {
      while (taskQueue.length > 0) {
          const task = taskQueue.shift();
          task();
      }
  };
  const isArray = (obj) => {
      return Object.prototype.toString.call(obj) === "[object Array]";
  };
  const node = document.createTextNode("");
  const observer = new MutationObserver(() => {
      runTask();
  });
  observer.observe(node, { characterData: true });
  return {
      push(tasks) {
          tasks = isArray(tasks) ? tasks : [tasks];
          taskQueue.push(...tasks);
          node.textContent = node.textContent === "0" ? "1" : "0";
      },
  };
})();

function a() {
    console.log("a");
}
function b() {
    console.log("b");
}
setTimeout(() => {
    console.log("c");
}, 0);
var p = new Promise((resolve) => {
    console.log("d");
    resolve();
});
schedule.push([a, b]); 
// output : d a b c