手写一个Promise

最新推荐文章于 2025-09-07 17:00:17 发布

转载最新推荐文章于 2025-09-07 17:00:17 发布 · 151 阅读

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CC 4.0 BY-SA版权

原文链接：https://juejin.im/post/5c54fc7b5188256283252eab

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#面试

本文详细介绍了如何从零开始手写一个简易版的Promise对象，包括then、catch等核心方法的实现，以及如何支持链式调用。通过阅读本文，读者可以深入理解Promise的工作原理。

相信很多人面试的时候都被问过：

你能手写一个Promise么？

肯定可以呀，手写没问题，不要求脚写就ok。

不多逼逼，show you the code。

function Promise(executor) {
  this._status = "Pending";
  this._subPromiseResolves = [];
  this._subPromiseRejects = [];
  this._value = undefined;
  let resolve = (val) => {
    if(this._status !=== "Pending") return
    this._status = "Resolved";
    this._value = val;
    this._subPromiseResolves.forEach((resolve) => {
      resolve(val);
    })
  }
  let reject = (reason) => {
    if(this._status !=== "Pending") return
    this._status = "Rejected";
    this._value = reason;
    this._subPromiseRejects.forEach((reject) => {
      reject(reason);
    })
  }
  executor(resolve, reject);
}
Promise.resolve = (val) => {
  return new Promise((resolve) => {
    resolve(val);
  })
}
Promise.reject = (val) => {
  return new Promise((resolve, reject) => {
    reject(val);
  })
}
Promise.prototype.then = function(onResolved, onRejected) {
  let newPromise;
  switch(this._status) {
    case "Resolved": 
      if(onResolved) {
        newPromise = Promise.resolve(onResolved(this._value));
      }else {
        //值穿透
        newPromise = Promise.resolve(this._value);
      }
      break;
    case "Rejected":
      if(onRejected) {
        newPromise = Promise.resolve(onRejected(this._value));
      }else {
        //错误往下冒泡
        newPromise = Promise.reject(this._value);
      }
      break;
    case "Pending":
      newPromise = new Promise((resolve, reject) => {
        //给前一个Promise存一个能触发回调并改变状态的函数
        this._subPromiseResolves.push(function(val){
          resolve(onResolved(val));
        });
        this._subPromiseRejects.push(function(val){
          reject(onRejected(val));
        });
      })
  }
  return newPromise
}
Promise.prototype.catch = function(onRejected){
  return this.then(null, onRejected);
}
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