Asynchronous "for" loop when there is no array

Question

Like the OP from this question, I want to do a for loop, and do something when all the actions have finished.

I checked the answer, and the async library, but all the solutions involve iterating over an array. I don't want to do something "forEach" element of an array, I don't have an array.

What if I just want to do an operation n times ? For example, say I want to insert n random entries in my database, and do something afterwards ? For now I'm stuck with something like :

function insertMultipleRandomEntries(n_entries,callback){
    var sync_i=0;
    for(var i=0;i<n_entries;i++){
        insertRandomEntry(function(){
            if(sync_i==(max-1)){
                thingDoneAtTheEnd();
                callback(); //watched by another function, do my stuff there
            }
            else{
                sync_i++;
                console.log(sync_i+" entries done successfully");
                thingDoneEachTime();
            }
        });
    }
}

Which is absolutely horrendous. I can't find anything like a simple for in async, how would you have done this ?

marinabilles · Accepted Answer · 2017-01-24 16:54:14Z

2

You can use Promises, supported without a library in node.js since version 4.0.

If the callback function of insertRandomEntry has a parameter, you can pass it to resolve. In the function given to then, you receive an array of parameters given to resolve.

function insertMultipleRandomEntries(n_entries,callback){
    var promises = [];
    for(var i=0;i<n_entries;i++) {
        promises.push(new Promise(function (resolve, reject) {
          insertRandomEntry(function (val) {
            thingDoneEachTime(val);
            resolve(val);
          });
        }));
    }
  
    Promise.all(promises).then(function (vals) {
      // vals is an array of values given to individual resolve calls
      thingDoneAtTheEnd();
      callback();
    });
}

edited Jan 24, 2017 at 16:54

answered Jan 24, 2017 at 16:22

marinabilles

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2 Comments

Teleporting Goat Over a year ago

Thanks. There's something done at every iteration, I wrote only console.log in my example so it made it seem unimportant but it's not, I edited the question. Where does it go ? I need the callback from insertRandomEntry, does it replace resolve or does it go after it ? If I could I'd also like to do something at the end. If I can't it's no big deal, I can still do it after catching the callback but it makes more sense do to in inside the function.

marinabilles Over a year ago

I updated the post to show how additional functions can be called before resolving the promise, and how to collect value results of individual promises.

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