手寫Promise A+ 規範

• 2020 年 2 月 18 日
• 筆記

基於ES6語法手寫promise A+ 規範，源碼實現

class Promise {      constructor(excutorCallBack) {          this.status = 'pending';          this.value = undefined;          this.fulfilledAry = [];          this.rejectedAry = [];            //=>執行EXCUTOR（異常捕獲）          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) {              //=>有異常信息按照REJECTED狀態處理              rejectFn(err);          }      }        then(fulfilledCallBack, rejectedCallBack) {          //=>處理不傳遞的狀況          typeof fulfilledCallBack !== 'function' ? fulfilledCallBack = result => result : null;          typeof rejectedCallBack !== 'function' ? rejectedCallBack = reason => {              throw new Error(reason instanceof Error ? reason.message : reason);          } : null;            //=>返回一個新的PROMISE實例          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:記錄成功的數量 RESULT:記錄成功的結果              let index = 0,                  result = [];              for (let i = 0; i < promiseAry.length; i++) {                  //=>promiseAry[i]:                  //每一個需要處理的PROMISE實例                  promiseAry[i].then(val => {                      index++;                      result[i] = val;//=>索引需要和promiseAry對應上，保證結果的順序和數組順序一致                      if (index === promiseAry.length) {                          resolve(result);                      }                  }, reject);              }          });      }  }    module.exports = Promise;