手写Promise A+ 规范

• 2020 年 2 月 18 日
• 筆記

基于ES6语法手写promise A+ 规范，源码实现

class Promise {      constructor(excutorCallBack) {          this.status = 'pending';          this.value = undefined;          this.fulfilledAry = [];          this.rejectedAry = [];            //=>执行EXCUTOR（异常捕获）          let resolveFn = result => {              let timer = setTimeout(() => {                  clearTimeout(timer);                  if (this.status !== 'pending') return;                  this.status = 'fulfilled';                  this.value = result;                  this.fulfilledAry.forEach(item => item(this.value));              }, 0);          };          let rejectFn = reason => {              let timer = setTimeout(() => {                  clearTimeout(timer);                  if (this.status !== 'pending') return;                  this.status = 'rejected';                  this.value = reason;                  this.rejectedAry.forEach(item => item(this.value));              }, 0);          };          try {              excutorCallBack(resolveFn, rejectFn);          } catch (err) {              //=>有异常信息按照REJECTED状态处理              rejectFn(err);          }      }        then(fulfilledCallBack, rejectedCallBack) {          //=>处理不传递的状况          typeof fulfilledCallBack !== 'function' ? fulfilledCallBack = result => result : null;          typeof rejectedCallBack !== 'function' ? rejectedCallBack = reason => {              throw new Error(reason instanceof Error ? reason.message : reason);          } : null;            //=>返回一个新的PROMISE实例          return new Promise((resolve, reject) => {              this.fulfilledAry.push(() => {                  try {                      let x = fulfilledCallBack(this.value);                      x instanceof Promise ? x.then(resolve, reject) : resolve(x);                  } catch (err) {                      reject(err);                  }              });              this.rejectedAry.push(() => {                  try {                      let x = rejectedCallBack(this.value);                      x instanceof Promise ? x.then(resolve, reject) : resolve(x);                  } catch (err) {                      reject(err);                  }              });          });      }        catch(rejectedCallBack) {          return this.then(null, rejectedCallBack);      }        static all(promiseAry = []) {//=>Promise.all()          return new Promise((resolve, reject) => {              //=>INDEX:记录成功的数量 RESULT:记录成功的结果              let index = 0,                  result = [];              for (let i = 0; i < promiseAry.length; i++) {                  //=>promiseAry[i]:                  //每一个需要处理的PROMISE实例                  promiseAry[i].then(val => {                      index++;                      result[i] = val;//=>索引需要和promiseAry对应上，保证结果的顺序和数组顺序一致                      if (index === promiseAry.length) {                          resolve(result);                      }                  }, reject);              }          });      }  }    module.exports = Promise;