Why did the “buy” button on the product on opencart stop working?

Question

JavaScript

I install my layout on opencart. I am slowly changing the default template in header.tpl. Initially, the "buy" button worked, but at some point it stopped, I did not have time to notice because of what. In the console writes:

Uncaught TypeError: $.ajax is not a function
at Object.add (common.js:151)
at HTMLButtonElement.onclick (index.php?route=common/home:319)

The button itself is:

<button type="button" onclick="cart.add('40');"><i class="fa fa-shopping-cart"></i> <span class="hidden-xs hidden-sm hidden-md">Купить</span></button>

In common.js on line 151:

$.ajax({
      url: 'index.php?route=checkout/cart/add',
      type: 'post',
      data: 'product_id=' + product_id + '&quantity=' + (typeof(quantity) != 'undefined' ? quantity : 1),
      dataType: 'json',
      beforeSend: function() {
        $('#cart > button').button('loading');
      },
      complete: function() {
        $('#cart > button').button('reset');
      },
      success: function(json) {
        $('.alert, .text-danger').remove();

        if (json['redirect']) {
          location = json['redirect'];
        }

        if (json['success']) {
          $('#content').parent().before('<div class="alert alert-success"><i class="fa fa-check-circle"></i> ' + json['success'] + ' <button type="button" class="close" data-dismiss="alert">&times;</button></div>');

          // Need to set timeout otherwise it wont update the total
          setTimeout(function () {
            $('#cart > button').html('<span id="cart-total"><i class="fa fa-shopping-cart"></i> ' + json['total'] + '</span>');
          }, 100);

          $('html, body').animate({ scrollTop: 0 }, 'slow');

          $('#cart > ul').load('index.php?route=common/cart/info ul li');
        }
      },
      error: function(xhr, ajaxOptions, thrownError) {
        alert(thrownError + "\r\n" + xhr.statusText + "\r\n" + xhr.responseText);
      }
    });

Answer 1

something tells me that you don't have jQuery library connected