动态创建承诺并按顺序运行

Question

I'm running into the following problem.

I have the following promise (simplified):

module.exports.checkVotes = (groupId) =>{
    return new Promise((resolve, reject)=>{
     // some db stuff onoing
     ...
     .then((votes)=>{return resolve(votes}})
     .catch((err)=>{return reject(err}})
    })
}

At some point I'm looping through an object. For each entry i have to call promise above. But before the 2. Promise starts, the first one have to be finished...and so on. I tried this, but as soon as I call the promise it gets executed.

 .then(()=>{
      for (let i=0;i<groups.length;i++){
           // for each group I want to call the promise later
           //but it executes as soon as I push it.
           tasklist.push(this.checkVotes(groups[i])) 
       }

       return tasklist.reduce((promiseChain, currentTask) => {
           return promiseChain.then(chainResults =>
               currentTask.then(currentResult =>
                    [ ...chainResults, currentResult ]
                )
           );
        }, Promise.resolve([])).then(arrayOfResults => {
                console.log(arrayOfResults)
            }).catch((err) =>{console.log(err)});
         })
  })

I can not run this with Promise.all() because for some database things, I need this to run sequentially. Moreover I can not hardcode this because the number of groups is variable.

How can I solve this?

Thanks for helping

Answer 1

Your problem was with putting promises in the taskList , not tasks (functions that return promises). The idea behind making them run in sequence is to call them in the then callback in the reduce :

return groups.reduce((promiseChain, currentGroup) => {
    return promiseChain.then(chainResults =>
        this.checkVotes(currentGroup).then(currentResult => [...chainResults, currentResult])
//      ^^^^^^^^^^^^^^^
    );
}, Promise.resolve([]))

But if you can use async / await syntax, you don't need to do all of this anyway and can write the much simpler and more efficient

const results = [];
for (const group of groups)
    results.push(await this.checkVotes(group));

Answer 2

seems to work like this:

groupIds = [12, 45, 34];
return groupIds .reduce((promise, groupId) => {
                return promise.then(() => {
                    return this.checkVotes(groupId)
                        .then((votes)=>{
                            votesList[groupId] = votes
                        })
                        .catch((err)=>{
                            throw err
                        })
                })
            }, Promise.resolve())