promise.ts

class Promise2 {
  state = "pending";
  callbacks = [];
  resolve(result) {
    if (this.state !== "pending") return;
    this.state = "fulfilled";
    setTimeout(() => {
      this.callbacks.forEach((handle) => {
        typeof handle[0] === "function" && handle[0](result);
      });
    }, 0);
  }
  reject(reason) {
    if (this.state !== "pending") return;
    this.state = "rejected";
    setTimeout(() => {
      this.callbacks.forEach((handle) => {
        typeof handle[1] === "function" && handle[1](reason);
      });
    }, 0);
  }
  constructor(fn) {
    if (typeof fn === "function") {
      fn(this.resolve, this.reject);
    } else {
      throw new Error("Not a function");
    }
  }
  then(succeed?, fail?) {
    const handle = [];
    if (typeof succeed === "function") {
      handle[0] = succeed;
    }
    if (typeof fail === "function") {
      handle[1] = fail;
    }
    this.callbacks.push(handle);
  }
}

export default Promise2;

function nextTick(fn) {
  if (process !== undefined && typeof process.nextTick === "function") {
    return process.nextTick(fn);
  } else {
    var counter = 1;
    var observer = new MutationObserver(fn);
    var textNode = document.createTextNode(String(counter));

    observer.observe(textNode, {
      characterData: true,
    });

    counter = counter + 1;
    textNode.data = String(counter);
  }
}