Promise

手写Promise

const PENDING = "pending";
const FULFILLED = "fulfilled";
const REJECTED = "rejected";

//微队列
function runMicrotasks(callback) {
  //判断node环境
  if (process && process.nextTick) {
    process.nextTick(callback);
  } else if (MutationObserver) {
    const p = document.createElement("p");
    const observer = new MutationObserver(callback);
    observer.observe(p, { childList: true });
    p.innerHTML = 1;
  } else {
    setTimeout(callback, 0);
  }
}

function isPromise(obj) {
  return (
    !!obj && //有实际含义的变量才执行方法，变量null，undefined和''空串都为false
    (typeof obj === "object" || typeof obj === "function") && // 初始promise 或 promise.then返回的
    typeof obj.then === "function"
  );
}

class MyPromise {
  /**
   * 创建一个Promise
   *@param {function}executor 任务执行器，立即执行
   */
  constructor(executor) {
    this._state = PENDING;
    this._value = undefined;
    this._handlers = [];
    try {
      executor(this._resolve.bind(this), this._reject.bind(this));
    } catch (error) {
      this._reject(error);
    }
  }
  _changeState(newState, value) {
    if (this._state !== PENDING) {
      return;
    }
    this._state = newState;
    this._value = value;
    this._runHandlers();
  }
  _pushHandler(executor, state, resolve, reject) {
    this._handlers.push({
      executor,
      state,
      resolve,
      reject,
    });
  }
  //处理每一个handler
  _runOneHandler(handler) {
    runMicrotasks(() => {
      if (this._state !== state) {
        return;
      }
      if (typeof executor !== "function") {
        this._state === FULFILLED ? resolve(this._value) : reject(this._value);
        return;
      }
      try {
        if (isPromise(result)) {
          result.then(resolve, reject);
        } else {
          resolve(result);
        }
      } catch (error) {
        reject(error);
      }
    });
  }
  //根据实际情况，执行队列
  _runHandlers() {
    if (this._state === PENDING) {
      return;
    }
    while (this._handlers[0]) {
      const handler = this._handlers[0];
      this._runOneHandler(handler);
      this._handlers.shift();
    }
  }
  then(onFulfilled, onRejected) {
    return new myPromise((resolve, reject) => {
      this._pushHandler(onFulfilled, FULFILLED, resolve, reject);
      this._pushHandler(onRejected, REJECT, resolve, reject);
      this._runHandlers();
    });
  }

  _resolve(data) {
    this._changeState(FULFILLED, data);
  }
  _reject(reason) {
    this._changeState(REJECTED, reason);
  }
}
const pro = new Promise((resolve, reject) => {
  resolve(1);
});
console.log(pro);