JavaScript之串行Promise实现

1. 问题：Promise原生语法，then的链式调用实现的promise串行执行不太语义化(声明式)，语法稍微有点繁杂；

   javascript

   new Promise((resolve, reject) => {
       // Promise #1
       resolve();
   })
       .then(result => {
           // Promise #2
           return result;
       })
       .then(result => {
           // Promise #3
           return result;
       });

2. 解决方法：a. reduce；b. async/await；

   javascript

   // 1. reduce
   const runPromiseByQueue = (promises) => {
       promises.reduce(
           (previousPromise, nextPromise) => previousPromise.then(() => nextPromise()),
           Promise.resolve()
       )
   }
   
   // 2. async/await
   const runPromiseByQueue = async(promises) => {
       for (let elem of promises) {
           await elem();
       }
   }
   
   // 使用
   function createPromise(time, id) {
       return () => new Promise(resolve =>
           setTimeout(() => {
               console.log('promise ', id);
               resolve();
           }, time)
       )
   }
   runPromiseByQueue([
       createPromise(300, 1),
       createPromise(200, 2),
       createPromise(100, 3),
   ])