Javascript Value Assignment Delay?

Question

I'm at a loss right now. I'm using a simple variable, whose value is assigned during a loop. After exiting the loop, the value of the variable is still undefined, unless I alert it's value first. Then everything works fine. What's going on here?

$(myarray).each(function(idx, item)
{
     fetchSomethingRemotely( success: function(data) {
           item.someValue = data; });

     // if the following alert is not there, doSomething will never get called
     // and the alert after the else will show item.someValue as undefined.
     alert(item.someValue);

     if (item.someValue != null) { doSomething(item.someValue); }
     else { alert(item.someValue); }

});

Edit:

Okay, so I've got a better handle in this now. The value assignment (item.someValue=123) happens inside of a callback function within this iteration. So the value is probably not there yet when I serially try to access it a couple of code lines below. How could I wait for the value to be assigned?

Answer 1

Did you remember to use var to define the variable. Are you sure the variable exists in the scopes you are using it. If you do for ( var i ... then it will only exist in the for scope, not outside it. You can use Webkit (Chrome, Safari)'s Developer Tools to debug your script by setting a breakpoint on the problem line, and then in the right column you can see all variables defined in the related scopes.

Answer 2

How could I wait for the value to be assigned?

The answer is already in your code. Just move doSomething into the callback function.

fetchSomethingRemotely( { success: function(data) {
       item.someValue = data;
       if (item.someValue != null) doSomething(item.someValue);
} });

Note that this will still move on to the next item before the current item has got its value. If you must perform all of the iteration sequentially, you can do something like this:

function iterate(index) {
  var item = myarray[index];
  fetchSomethingRemotely( { success: function(data) {
    item.someValue = data;
    if (item.someValue != null) doSomething(item.someValue);
    if (index < myarray.length - 1) iterate(index + 1);
  } });
}

And then you would fire off the whole process with iterate(0).

Answer 3

"How could I wait for the value to be assigned?"

Welcome to asyncronous programming!

You're going to need to put everything in the callback, not just the variable assignment.