Promise实现队列

有时候我不希望所有动作一起发生，而是按照一定顺序，逐个进行

 var promise=doSomething();

  promise=promise.then(doSomethingElse);

  promise=promise.then(doSomethingElse2);

  promise=promise.then(doSomethingElse2);

// forEach 实现

 function queue(things){

   let promise=Promise.resolve();

   things.forEach(element => {

    promise=promise.then(()=>{

      return new Promise(resolve=>{

        setTimeout(() => {

          console.log(element)

          resolve('ok');

        }, 1000);

      });

    })

   });

 }

  queue(['h1','h2','h3']);

// reduce 实现

 function queue2(things) {

        let promise = Promise.resolve();

        things.reduce(function (totel, item) {

            return totel.then(() => {

                return new Promise(resolve => {

                    setTimeout(() => {

                        console.log('reduce', item)

                        resolve('ok');

                    }, 1000);

                });

            });

        }, promise)

    }

    queue2(['h1', 'h2', 'h3']);