Referenceerror: $ Is Not Defined

ReferenceError: $ is not defined

Add jQuery library before your script which uses $ or jQuery so that $ can be identified in scripts.

<script src="http://code.jquery.com/jquery-1.11.0.min.js"></script>

Uncaught ReferenceError: $ is not defined problem

I think is becaus you have to import Jquery.
Place this in your HTML
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.5.1/jquery.min.js"></script>

How to solve this Uncaught ReferenceError: $ is not defined

It means that you tried to use Jquery in your Javascript Code without calling Jquery Library or the code is called without the library was fully loaded.

I notice :

That you haven't closed your script tag
You use Jquery so you can use $('#id_name') to select element by Id instead of document.getElementById('note_1')
Get element value by using Element.val() instead of Element.value

Try to edit your code like this

<?php
    $sql = "SELECT * FROM table WHERE a_column='certain_value'";
    if (mysqli_query($conn, $sql)) {
        $result = mysqli_query($conn, $sql);
        if (mysqli_num_rows($result) > 0) {
            while($row = mysqli_fetch_assoc($result)) {
                $note = $row["note"];
                $code = $row["code"];
            }
        }
    }
?>
<!DOCTYPE html>
<html>
    <head>
        <meta charset="UTF-8">
        <title>Some title</title>
    </head>
    <body>
        <form method="post" accept-charset="UTF-8">
            <input type='text' id='note_1' name='note_1' value=<?= $code ?> readonly>";
            <input type='text' id='new_note' name='new_note'>";
            <img src='icon_to_click.png' id='icon_to_click' name='icon_to_click' >";
        </form>
        <script>
            $(document).ready(function() {
                $('#icon_to_click').click(function() {
                    var note_orig = $('#note_1').val();
                    var code_val = '<?= $code ?>';
                    var note_new = $('#new_note').val();
                    if (note_new != note_orig) {
                        $.ajax({
                            type: 'POST',
                            url: 'update_notes.php',
                            data: {'code': code_val, 'note': note_new},
                            success: function(response){
                                $('#note_1').val() = note_new;
                            }
                        });
                    }
                });
            });
        </script>
    </body>
</html>

$.getJSON uncaught reference error $ is not defined

Add jquery library before your script

<script src="http://code.jquery.com/jquery-3.5.1.min.js"></script>

it shows uncaught referenceError :$ is not defined. whats the exact matter cant understand

Maybe because you are including the script with the type "application/json". Try to remove the type attribute on your script tag.

Console error Uncaught ReferenceError: $ is not defined

Your file can't be found. Try to replace your code from your current code to this.

<script src="http://ajax.googleapis.com/ajax/libs/jquery/3.4.1/jquery.min.js"></script>
<script src="http://cdn.datatables.net/1.10.7/js/jquery.dataTables.min.js"></script>

Getting Uncaught ReferenceError: $ is not defined for jquery

Check Handling code which relies on jQuery before jQuery is loaded . It will solve your problem. Because your jquery is loaded. So t is recommended to put external scripts imports before the closing body tag, it allows asynchronous loading while the loading in the head tag is a blocking synchronous loading.

Referenceerror: $ Is Not Defined