How can I handle concurrency to have just 2 or 3 promises running at the same time?

Question

According to bluebird, the usage of bluebird seems quit simple. This is how I understand :

Promise.map(
    Iterable<any>|Promise<Iterable<any>> input,
    function(any item, int index, int length) mapper,
    [Object {concurrency: int=Infinity} options]
) -> Promise

What I understand is that I could have something like this :

var promisfunc = Promise.promisify(
  function func(val, cb){
    console.log(val);

    //process data with val ...  
    cb(err, processedData);
  }
);
Promise.map([1, 2, 3, 4, 5, 6, 7, 8, 9],
  function(d){
    return promisfunc(data);
  }, {concurrency: 1} 
).then(function(data){
  console.log(data); // array of data
})
.catch(
  function(err)
  {console.log(err)}
);

I have tried to make this work for hours without success. Every time I run all processes are fired at the same time. How can I handle concurrency to have just 2 or 3 promises running at the same time ? Can you explain whant I misunderstand here please ?

Answer 1

You have a fundamental misunderstanding of what Promise.map() expects in its arguments. It expects a standard function which is then promisify'd to return asynchronously.

let Promise = require('bluebird');

var promisfunc =  function func(element, index, length) {
    console.log(element);
    return element;
}

Promise.map([1, 2, 3, 4, 5, 6, 7, 8, 9], promisfunc)
.then( (data) => console.log(data) )
.catch( (err) => console.log(err) );

The most significant thing I did was get rid of the var promisfunc = Promise.promisify(function) because that's not what Promise.map() expects. It's replaced with a bog standard function that returns elements after logging them just like a standard map function.