Use jQuery in WordPress

WordPress has its own jQuery library. The point is using jQuery instead of $.

But you can map jQuery to $ in an anonymous function (IIFE pattern):

jQuery( document ).ready(function( $ ) {
    window.dummy_script.init();
});

(function( $ ) {
    'use strict';

    if ( window.dummy_script !== undefined ) {
        return;
    }

    window.dummy_script = {
        init: function() {
            // ...
        }
    }
})( jQuery );