Q:

Jquery animate fontSize back to original size on click

JSFIDDLE
$("button").click(function(){
$("h1").animate({fontSize: "50px"});

How to make the font size return to its original size if clicked again. 

A:

You can achieve this by checking the current font size and then toggle the value to set, like this:

$("button").click(function() {
  $("h1").animate({
    fontSize: $('h1').css('font-size') == '32px' ? "50px" : '32px'
  });
})
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<button>CLICK</button>
<h1>HELLO</h1>

That said, it would be better practice (and result in a smoother effect) to do the transition in CSS and just toggle the class on the element in your JS code, like this:

$("button").click(function() {
  $("h1").toggleClass('large');
})
h1 { transition: all 0.5s; }
.large { font-size: 50px; }
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<button>CLICK</button>
<h1>HELLO</h1>

