Javascript check if an input is a Long or not

Question

I have this problem: I have an input field like this:

<div class="sigle-sz">
  <span class="label-sz">44</span>

  <input id="fillOrderFormEntries0.sizeEntries0.sizeValues5" 
         name="fillOrderFormEntries[0].sizeEntries[0].sizeValues[5]" 
         type="tel" 
         value="">

  <div class=" available wtqtn yes ">78</div>
</div>

and I want check if the value that user input in is <= of the max Long value or not. How can I do and when? I have to do this when the event onFocus change? I have to do this in Javascript, capturing the event and make the check.

Thanks to all.