Javascript for loop wait for callback

Question

I have this function:

function tryStartLocalTrendsFetch(woeid) {
    var userIds = Object.keys(twitClientsMap);
    var isStarted = false;

    for (var i = 0; i < userIds.length; i++) {
        var userId = userIds[i];
        var twitClientData = twitClientsMap[userId];
        var isWoeidMatch = (woeid === twitClientData.woeid);
        if (isWoeidMatch) {

            startLocalTrendsFetch(woeid, twitClientData, function (err, data) {
                if (err) {
                    // Couldn't start local trends fetch for userId: and woeid:
                    isStarted = false;
                } else {
                    isStarted = true;
                }
            });
            // This will not obviously work because startLocalTrendsFetch method is async and will execute immediately
            if (isStarted) {
                break;
            }
        }
    }
    console.log("No users are fetching woeid: " + woeid);
}

The gist of this method is that I want the line if (isStarted) { break; } if (isStarted) { break; } to work. The reason is that if it's started it should not continue the loop and try to start another one.

I'm doing this in NodeJS.

Answer 1

try to use a recursive definition instead

function tryStartLocalTrendsFetch(woeid) {
  var userIds = Object.keys(twitClientsMap);
  recursiveDefinition (userIds, woeid);
}

function recursiveDefinition (userIds, woeid, userIndex)
  var userId = userIds[userIndex = userIndex || 0];
  var twitClientData = twitClientsMap[userId];
  var isWoeidMatch = (woeid === twitClientData.woeid);
  if (isWoeidMatch && userIndex<userIds.length) {
      startLocalTrendsFetch(woeid, twitClientData, function (err, data) {
          if (err) {
            recursiveDefinition(userIds, woeid, userIndex + 1)
          } else {
            console.log("No users are fetching woeid: " + woeid);
          }
      });
  } else {
    console.log("No users are fetching woeid: " + woeid);
  }
}

Answer 2

You may also use async ( npm install async ):

var async = require('async');

async.forEach(row, function(col, callback){
    // Do your magic here
    callback();  // indicates the end of loop - exit out of loop
    }, function(err){
        if(err) throw err;
    });

More material to help you out: Node.js - Using the async lib - async.foreach with object