execute a function against array items in sequence

Question

Say I have an array and a function A:

var array = ['a', 'b', 'c'];
function A(p) { ... };

now I want to pass each item of array to function A, and want them to be executed in a sequential order. usually there won't be any problem. I can do:

array.forEach(function(item) {
  A(item);
}

however, if there are some asynchronous action in A, everything becomes messed up. so the question is:

How can I have them executed orderly even if there are asynchronous actions in A?

By which I mean, I want A('b') to be executed after A('a') is completely finished (including all the asynchronous actions in there).

I guess there must have been some utilities doing such things already. can someone shed me some light?

Did you try to pass function A as parameter to foreEach instead of wrapping it? — user4164128
– user4164128, Commented Aug 17, 2015 at 9:35
In your current form, it's mostly impossible, as the function A only accept one param, and it has no ability to notify server when it complete its works(the callbacks), so the server can't decide whether A('b') is complete or not. — fuyushimoya
– fuyushimoya, Commented Aug 17, 2015 at 9:36
@fubbe I guess it runs the same way. there's no guarantee of the async actions in A. — yaoxing
– yaoxing, Commented Aug 17, 2015 at 9:38
@fuyushimoya oh, yes. it doesn't have to look like this. callback is acceptable. — yaoxing
– yaoxing, Commented Aug 17, 2015 at 9:40

thomas · Accepted Answer · 2015-08-17 09:40:20Z

6

This is a perfect use case for "eachSeries" from async.js

You could use it this way

async.eachSeries(array, A, function(err) {
    // this function gets called at the end 
    // after your array has been looped
});

And you would have to modify your A function with a second parameter - a callback.

function A(p, callback) {
    // ...
    return callback();
}

That calls the callback parameter once your async calls in A are finished.

answered Aug 17, 2015 at 9:40

thomas

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3 Comments

yaoxing Over a year ago

OK I'll try it. It's weird in the async document it mentioned this method, but there was no detail about this function. I thought it was an alias of each. I'll try and come back after a while.

yaoxing Over a year ago

OK this is what I'm looking for. thanks for the answer. and now I know why I missed this one. There was a note saying: <name>Series - the same as <name> but runs only a single async operation at a time I simply missed it.

thomas Over a year ago

They removed the detailed description a while ago and I think it was a bit easier to understand when it still was there.

Amit · Accepted Answer · 2015-08-17 10:01:19Z

3

JavaScript Promises allow you to do just that.

This example is pretty convoluted, but it shows some aspects of using Promises:

var arr = ['a', 'b', 'c'];

function processAsync(val) {
  return new Promise(function(res, rej) {
    console.log("Starting:", val);
    setTimeout(function () {
      console.log("Done:", val);
      res();
    }, 1000);
  });
}

arr.reduce(function(promise, i) {
  return promise.then(processAsync.bind(null, i));
}, Promise.resolve());

edited Aug 17, 2015 at 10:01

answered Aug 17, 2015 at 9:51

Amit

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2 Comments

sdgluck Over a year ago

I think this is a better solution if the OP is not already using async.

yaoxing Over a year ago

I was using async actually. but this is definitely an possible answer. thanks anyway.

shut up and stand up · Accepted Answer · 2015-08-17 10:03:03Z

0

var array = ['a', 'b', 'c'],i = 0;
var A = function(p){
    //some code
    $.ajax({
        success:function(){
            try{
                A(item[++i]);
            }catch{
                return;
            }
        }
    });
}

Are you mean to this？

answered Aug 17, 2015 at 10:03

shut up and stand up

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1 Comment

yaoxing Over a year ago

it's close, but I'm in server side with nodejs. anyway I've get the answer above. thanks for the help.

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