Ajax requests - $ is not defined

Question

I try to call my .php file (containing sql query for my database) from another .php file like the following:

...
<canvas id="myCanvas" width="600" height="100"></canvas>
        <script>
            function parseData(data) {
                console.log(data);
            }
            function mainFunction() {
                $.post('ajaxQueries/numberOfTrucksInWeek.php'), {}, function (data) {
                    if (typeof data === 'object') {
                        parseData(data);
                    }
                }
            }
            mainFunction();

When I run this code, I get the following error:

Uncaught ReferenceError: $ is not defined

And not sure why this happens, as the syntax of ajax requests I used above should be correct. Anyone see my mistake?