javascript conditional not reaching else

I'm working on creating a toggle function to 'favorite' an item from a list of many. I've got working script to toggle the item in and out of a user-specific favorites list, communicate that change to a database, and populate the rest of the site accordingly. That all works fine, it's mostly PHP and Ajax. However, my javascript is ass. I'm stuck on a conditional to change the icon from a filled heart to an empty one. For some reason it never reaches the else statement even when the if statement is false. If I reverse the conditions, it still handles the if fine but never the else.

the image is:

<img src="includes/icons/fave-<?php echo $favStatus; ?>.png" id="faveToggle" class="faveIcon" onClick="toggleFave()">

the conditional, located in toggleFave() is:

if(document.getElementById('faveToggle').src.toString().indexOf("fave-false.png")){
    document.getElementById('faveToggle').src = "includes/icons/fave-true.png";
} else {
    document.getElementById('faveToggle').src = "includes/icons/fave-false.png";
}

So, uhh, whuddo I do?