CODEWITHSUNDEEP
javascriptnode.jsloopscallback
Mithun Satheesh
Mithun Satheesh

Reputation: 27845

how to break async.js each loop?

Hi i am using async module of node.js for implementing a for loop asynchronously.

My question is: how to break the loop execution and get out of the loop? I tried giving return , return false but no luck.

Here is the sample code:

async.until( 

    function() { return goal; },

    function(callback) {

        async.each(_rules, 

            function(rule,callback) {

                var outcome = true;
                .... some code ....
                if(changes){ 
                      console.log("hi");
                      return false;// HERE I NEED TO BREAK
                } 
                else
                callback();

            }, 

            function(err){  }
        );

        if (!changes || session.process)                
            goal = true;

        callback();
    },

    function(err){ callback(session); }     
);

Upvotes: 11

Views: 21809

Answers (3)

João Pimentel Ferreira
João Pimentel Ferreira

Reputation: 16233

You have also async.detect

Returns the first value in coll that passes an async truth test. The iteratee is applied in parallel, meaning the first iteratee to return true will fire the detect callback with that result.

// asynchronous function that checks if a file exists
function fileExists(file, callback) {
   fs.access(file, fs.constants.F_OK, (err) => {
       callback(null, !err);
   });
}

async.detect(['file3.txt','file2.txt','dir1/file1.txt'], fileExists,
   function(err, result) {
       console.log(result);
       // dir1/file1.txt
       // result now equals the first file in the list that exists
   }
);

Upvotes: 0

user568109
user568109

Reputation: 47993

async.until repeatedly calls function until the test returns true. So test must return true so that you exit the loop. This is opposite of async.whilst which runs repeatedly while test evaluates to be true.

async.each calls the functions in parallel so what it returns does not matter. It is not a loop which you can break, but an iterator looping over the array. Your condition to stop using async.each should be in test for async.until and you should iterate over the rules yourself.

Upvotes: 7

AndyD
AndyD

Reputation: 5385

There isn't really a "loop" as such to break out of. All your items in your collection are used in parallel

The only way to "break" the "loop" is to call the callback with an error argument. As there is nothing to stop you from putting other things in there you could hack it a little bit.

From the docs:

Note, that since this function applies the iterator to each item in parallel there is no guarantee that the iterator functions will complete in order.

Even if you return an error, you will still have several outstanding requests potentially so you really want to limit the amount of items you use in one go. To limit the amount of outstanding requests, you could use eachSeries or eachLimit.

For example:

    async.each(_rules, 
        function(rule,callback) {
            if(changes){                       
                  return callback({ data: 'hi'}); // stop
            } 
            ...
            if(realerror){
               return callback({ error: realerror}); // stop with an error
            }
            callback(); // continue
        }, 

        function(result){
          if(!result){
            return othercallback('no results');
          }
          // check if we have a real error:
          if(result.error){
            return othercallback(result.error);
          }
          return othercallback(null, result.data);
        }
    );

PS: if you're not doing async, use underscore

Upvotes: 5

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