前言

百度了很多次别人写的promise ,也是一时兴起今天自己写了一个并且做了这么一个记录

Promises/A+

同学们:此处自行查阅Promises/A+规范

Just do it

接下来我根据我自己习惯看规范

“promise” is an object or function with a then method whose behavior conforms to this specification.

菜?自己的翻译:class Promise{}和function Promise{} 都行,那我写的是class

 class Promise{
        constructor(execuctor) {}
    }
 export default Promise
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A promise must be in one of three states: pending, fulfilled, or rejected….

菜?自己的翻译:每个promise 都有三个状态 ,且pedding状态时可变更(通过resolve和reject) fulfilled, or rejected,但只支持变更一次,不能转变成任何其他状态。

“reason” is a value that indicates why a promise was rejected.

“value” is any legal JavaScript value (including undefined, a thenable, or a promise).

菜?自己的翻译: resolve() 和reject() 的参数和其值的初始化,结合2做状态变更处理

//class 同级外层 定义状态的常量,等待使用,实际过程中为了可读性,通常都会定义常量
//状态常量
/**初始化状态*/
const PENDING = "PENDING";
/**成功状态*/
const FULFILLED = "FULFILLED";
/**失败状态*/
const REJECTED = "REJECTED";

 class Promise{
        constructor(execuctor) {
          this.status = PENDING;
          this.value = undefined;
          this.reason = undefined;
          const resolve = (val) => {
              // 状态变更后 不可以在更改为其他状态,只支持修改一次
              if (this.status === PENDING) {
                this.value = val; //成功状态时 value的赋值
                this.status = FULFILLED; //修改状态
              }
            };
          const reject = (err) => {
              // 状态变更后 不可以在更改为其他状态,只支持修改一次
              if (this.status === PENDING) {
                this.reason = err; //失败状态时的 reason的赋值
                this.status = REJECTED; //修改状态
              }
        }

    }
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“exception” is a value that is thrown using the throw statement.

菜?自己的翻译:throw 语句抛出的异常。这里按照我自己的理解用try,catch 捕获异常的时候调用reject(err)

try {
      execuctor(resolve, reject);
    } catch (err) {
      reject(err);
    }
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A promise’s then method accepts two arguments:
promise.then(onFulfilled, onRejected)….

菜?自己的翻译:then 方法的支持,FULFILLED执行onFulfilled,REJECTED执行onRejected而且参数均得是函数,这里规范我们没有全部粘贴

then(onFulfilled, onRejected) {
    if (this.status === FULFILLED) {
      onFulfilled(this.value);
    }
    if (this.status === REJECTED) {
      onRejected(this.reason);
    }
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完整代码

class Promise {
  constructor(execuctor) {
    this.status = PENDING;
    this.value = undefined;
    this.reason = undefined;
    const resolve = (val) => {
      // 状态变更后 不可以在更改为其他状态,只支持修改一次
      if (this.status === PENDING) {
        this.value = val; //成功状态时 value的赋值
        this.status = FULFILLED; //修改状态
      }
    };
    const reject = (err) => {
      // 状态变更后 不可以在更改为其他状态,只支持修改一次
      if (this.status === PENDING) {
        this.reason = err; //失败状态时的 reason的赋值
        this.status = REJECTED; //修改状态
      }
    };
    try {
      execuctor(resolve, reject);
    } catch (err) {
      reject(err);
    }
  }
  then(onFulfilled, onRejected) {
  //then 的参数 resolve函数,和reject函数
    if (this.status === FULFILLED) {
      onFulfilled(this.value);
    }
    if (this.status === REJECTED) {
      onRejected(this.reason);
    }
  }
}
 export default Promise

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我们测试结果测试插件用的是 code run (按照指南VS code插件分享之 — Run Code)