If checkbox is checked

I want to be able to change the var chars according to the checkboxes checked.

Right now, I was looking around for some if checkbox is checked stuff, but when ever I attempt it, the password comes up blank.

function randomString() {
var chars = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXTZabcdefghiklmnopqrstuvwxyz=-][';/.,`~!@#$%^&*()_+|}{:?><";
var string_length = 10;
var randomstring = '';
for (var i=0; i<string_length; i++) {
    var rnum = Math.floor(Math.random() * chars.length);
    randomstring += chars.substring(rnum,rnum+1);
}
if($('#checkbox1').is(':checked')){
    var lowstring = randomstring.toLowerCase();
    document.randform.randomfield.value = lowstring;
} else {
    document.randform.randomfield.value = randomstring;
}

//  document.randform.randomfield.value = randomstring; //
}



    <div class="preference">
  <div class="form2">


     <div class="wanted"> <!-- Toggle Special Characters -->
      <label class="input-toggle">

       <input type="checkbox" id="checkbox1" checked>
        <span></span>
          </label>
          <p>Toggle Special Characters</p>


    <div class="wanted"> <!-- Toggle Numbers -->
      <label class="input-toggle">
       <input type="checkbox" checked>
        <span></span>
          </label>

    <p>Toggle Numbers</p>

          <div class="wanted"> <!-- Toggle Uppercase -->
      <label class="input-toggle">
       <input type="checkbox" checked>
        <span></span>
          </label>

   <p>Toggle Uppercase</p>

          <div class="wanted"> <!-- Toggle Lowercase -->
      <label class="input-toggle">
       <input type="checkbox" checked>
        <span></span>
          </label>
    <p>Toggle Lowercase</p>
  </div>
</div>