Select All Checkboxes - Toggle On and Off

I have a table with a checkbox in the first column - when checked this performs an AJAX script that updates a PHP session variable with the selected values. I also have a check box at the top of the first column to allow the user to select all items in the table and perform an AJAX script.

I have this working for the "select all when not checked" state, i.e. when the page first loads. I now need to add the reverse state - unchecking all checkboxes on the page.

Here's how my table looks and the script that runs when you click the checkbox in the table header:

$(document).ready(function() {
  $('.select-all').on('click', function() {
    var values = []; // will contain all checkbox values that you can send via ajax
    $('table > tbody input[type="checkbox"]').each(function(i, el) {
      $(el).prop('checked', true);
      values.push(el.value);
    });
    var productIDs = values.join();;
    // Create a reference to $(this) here:
    $this = $(this);
    $.post('productSelections.php', {
      type: 'updateSelections',
      productIDs: productIDs,
      selectionType: 'single'
    }, function(data) {
      data = JSON.parse(data);
      if (data.error) {
        var ajaxError = (data.text);
        var errorAlert = 'There was an error updating the Product Selections Selections - ' + ajaxError;
        $this.closest('td').addClass("danger");
        //display AJAX error details
        $("#updateSelectionsErrorMessage").html(errorAlert);
        $("#updateSelectionsError").show();
        return; // stop executing this function any further
      } else {
        $this.closest('td').addClass("success")
        $this.closest('td').removeClass("danger");
      }

    }).fail(function(xhr) {
      var httpStatus = (xhr.status);
      var ajaxError = 'There was an error updating the Product Selections - AJAX request error. HTTP Status: ' + httpStatus + '. Please contact Apple Business Operations';
      $this.closest('td').addClass("danger");
      $("#updateSelectionsErrorMessage").html(ajaxError);
      $("#updateSelectionsError").show();
      $this.attr('checked', false); // Unchecks it
    });
  });
});

<script src="http://ift.tt/1oMJErh"></script>
<table class="table table-condensed table-striped table-bordered">
  <thead>
    <th><input type="checkbox" class="select-all checkbox" name="select-all" /></th>
    <th class="text-center" scope="col">Product ID</th>
    <th class="text-center" scope="col">Description</th>
  </thead>
  <tbody>

    <tr class="" id="85799">
      <td id="AT36288"><input type="checkbox" class="select-item checkbox" name="select-item" value="AT36288" /></td>
      <td>AT36288</td>
      <td>Apples</td>
      <td></td>
    </tr>
    <tr class="" id="85800">
      <td id="AT36289"><input type="checkbox" class="select-item checkbox" name="select-item" value="AT36289" /></td>
      <td>AT36289</td>
      <td>Bananas</td>
      <td></td>
    </tr>
    <tr class="" id="85801">
      <td id="AT36290"><input type="checkbox" class="select-item checkbox" name="select-item" value="AT36290" /></td>
      <td>AT36290</td>
      <td>Oranges</td>
      <td></td>
    </tr>
    <tr class="" id="85803">
      <td id="AT36292"><input type="checkbox" class="select-item checkbox" name="select-item" value="AT36292" /></td>
      <td>AT36292</td>
      <td>Grapes</td>
      <td></td>
    </tr>
  </tbody>
</table>

I just can't work out what to change to uncheck all checkboxes if they are currently checked?