javascript - Validate input field, display error on submit and stop dialog open if error -

i have problem trying validate input field , alert error, if number below 50, know how display error popup (alert, or ever), stop opening dialog modal if error occurred.

i did script force users write numbers below 50 with:

<script>     function handlechange(input) {         if (input.value < 50) input.value = 50;         if (input.value > 1000000) input.value = 1000000;     } </script> 

but once user type, example: 23 , click on submit, it'll change number in background 50, not aware of that... can it?

here's full code i'm using:

<form id="testconfirmjq" name="testconfirmjq" method="post" onsubmit="return validator(users)" action="output.php">     <label>     <select id="selectusers" class=" outtahere" name="users" onchange='choice();'>         <option value="0" selected="selected">choose...</option>         <option value="1">test</option>     </select>     </label>      <input type="hidden" id="ids" name="ids" >     <input type="hidden" id="use" name="username" >     <input type="hidden" id="ful" name="full_name" >      <p>         how much?         <input type="text" id="quantity" name="quantity" onchange="handlechange(this);">         <div style="font-size: 12px; float: left;">*minimum quantity: 50</div>     </p>     <input id="submitjq" name="submitjq" type="submit" class="styled-button-1" value="send" /> </form>  <div id="dialog" title="potvrdite">     <div class="scroll-content">         <ul class="countrylist">             <img src="./images/png.png" class="vizitke"/>         </ul>     </div> </div> 

thanks in advance...

<script> function handlechange(input)   {     if (input.value < 50)      {        alert('your alert');        input.value = 50;      }       if (input.value > 1000000)       {        alert('your alert');          input.value = 1000000;       }      return false;  }  </script> 

i looking for