Calling Helper Within If Block in Handlebars Template

I am working with Handlebars.js template engine and am trying to figure out a way to do something like this (contrived example):

{{#if itemSelected "SomeItem"}}
    <div>This was selected</div>
{{/if}

where itemSelected is a registered helper like this:

Handlebars.registerHelper("itemSelected", function(item) {
    var selected = false;
    // Lots of logic that determines if item is selected
    return selected;
});

I get errors when trying to use this syntax for the template, and I cannot find any example showing this kind of thing. I do see simple #if blocks like this...

{{#if myValueInContext}}
    <div>This will show if myValueInContext results in a truthy value.</div>
{{/if}}

But, I can't figure out how to tackle the first example. Maybe I am approaching this wrong.

By the way, I tagged this Mustache as I could not add a Handlebars tag to the question.

I don't think this is going to work. If I understand the handlebars documentation correct, the #if is a registered block-helper itself and does not take another registered helper as an argument.

According to the documentation you might implement it like that

Handlebars.registerHelper('ifItemSelected', function(item, block) {
  var selected = false;
  // lots of logic that determines if item is selected

  if(selected) {
    return block(this);
  }
});

Afterwards you should be able to call it with

{{#ifItemSelected SomeItem}}
    This was selected
{{/ifItemSelected}

but you have to make sure SomeItem has the proper format. I don't see a way to use a registered handler as conditional in an if-statement.