Javascript:worker Synchronization

I am working on HTML5 web worker and I made a function that spawn few workers and return the result, but problem is that it returns the value before the worker updates the result.

Solution 1:

Like the comment points out - Web Workers work asynchronously (think of how AJAX works)

You can use an asynchronous semaphore to yield only when they're all done

functiondoSomething(callback){
    var counter = array1_rows;
    for (i = 0; i < array1_rows; i++)
    {
        var worker = newWorker('json.js');
        worker.postMessage(arr1[i]);
        worker.postMessage(arr2);
        worker.postMessage(i);

        worker.onmessage = function(){
            storeResult.apply(arguments);//call it the same way it was called before.
            counter--;
            if(counter === 0){ // all workers returnedcallback(result); //or whatever you're updating in storeResult
            }
        };
    }
}

Now you can call it like:

doSomething(function(result){
    console.log(result); //this will print the correct result.
});

For more information about how JS async operations work I recommend this ajax related question that includes a description of the problem and how to approach it.,