动手写一个符合promises-aplus-tests测试的promise库

[sevenCon]

前言

动手写一写平时用到的Promise库, 早些时候,早就有动手写一写念头了, 如果真要动手写一个通过promise-aplus-tests的Promise库, 还是得动动心思的,里面有很多细节, 需要花很多心思去理解里面的test-case, 这部分的东西在官网是没有的.

promise的原理也不难,就是在then的时候,收集回调函数的依赖. 在resolve, 或者reject被调用的时候, 去一个接一个的调用then收集的回调函数.

流程图

简单的promise

const PENDING = 'pending';
const RESOLVED = 'resolved';
const REJECTED = 'rejected';

function Promise(callback) {
  this.value = '';
  this.reason = '';
  this.onRejectedCallbacks = [];
  this.onResolveCallbaccks = [];
  this.status = PENDING;
  let self = this;

  function resolve(value) {
    if (self.status == PENDING) {
      self.status = RESOLVED;
      self.value = value;
      self.onResolveCallbaccks.forEach(fn => {
        fn();
      });
    }
  }
  function reject(reason) {
    if (self.status == PENDING) {
      self.reason = reason;
      self.status = REJECTED;
      self.onRejectedCallbacks.forEach(fn => {
        fn();
      });
    }
  }
  callback(resolve, reject);
}

function resolvePromise(promise2, x, resolve, reject) {
  if ((typeof x == 'object' && x !== null) || typeof x == 'function') {
    let then = x.then;
    if (typeof then === 'function') {
      then.call(
        x,
        v => {
          resolvePromise(promise2, v, resolve, reject);
        },
        r => {
          reject(r);
        }
      );
    } else {
      resolve(x);
    }
  } else {
    resolve(x);
  }
}

Promise.prototype.then = function(onFulfilled, onRejected) {
  let self = this;
  let promise2 = new Promise(function(resolve, reject) {
    if (self.status == RESOLVED) {
      setTimeout(function() {
        let x = onFulfilled(self.value);
        resolvePromise(promise2, x, resolve, reject);
      });
    }
    if (self.status == REJECTED) {
      setTimeout(function() {
        let x = onRejected(self.reason);
        resolvePromise(promise2, x, resolve, reject);
      });
    }
    if (self.status == PENDING) {
      self.onResolveCallbaccks.push(function() {
        setTimeout(function() {
          let x = onFulfilled(self.value);
          resolvePromise(promise2, x, resolve, reject);
        });
      });

      self.onRejectedCallbacks.push(function() {
        setTimeout(function() {
          let x = onRejected(self.reason);
          resolvePromise(promise2, x, resolve, reject);
        });
      });
    }
  });

  return promise2;
};

module.exports = Promise;

以上的主流程是可以简单的运用的, 但是对于通过promises-aplus-tests的, 还需要检验很多的判断流程

• 必要的try...catch, 在then函数的,构造函数的回调, 必须添加try...catch
• resolve函数有可能返回一个promise
• Promise的catch函数, 同样需要返回一个promise
• then 函数的参数的缺省值, 需要返回下一个promise
• resolvePromise 的x.then不可获取多次, 因为可能被getter挟持, 直接返回Error

以上是一个基本的注意事项, 但是还有许许多多的要点, 需要再执行的时候去看test-case的代码, 才知晓怎么改.
此外还需要了解一下Promises/A+规范 的详情.
本人写了一个完整版的Promise在promise-demo

参考

https://promisesaplus.com/#point-21
本文的github地址在动手写一个符合promises-aplus-tests测试的promise库, 如有侵权或其他问题,请issue留言, 感谢