根据选择的选项添加或删除输入字段的必需属性

Question

Basically I have a form in which a certain amount of input fields and selections (ingredient, quantity, unit of measure) are generated by a loop. Now I would like to remove the attribute required from the quantity field only when the associated selected unit of measure is 'je'

<form method="post">

    <?php
        for ($n = 0; $n < $num; $n++) {

            $data = $DB->query("SELECT * FROM ingredient"); ?>
            <select name="list_ingr[]" required> <?php

            while ($ingrs = $data->fetch()) { ?>

                <option value="<?php echo $ingrs['id_ingr'] ?>"><?php echo $ingrs['name'] ?></option> - <?php
            } ?>

            </select>

            Qt. <input type="number" name="list_quant[]" step="1" min="1" required> 

            <select name="measure[]">
                <option value="none"></option>
                <option value="gr">gr</option>
                <option value="ml">ml</option>
                <option value="j.e.">j.e.</option>
            </select><br>

        <?php    
        }
    ?>
    <br>
    <input type="submit" name="confirm" value="Confirm">

</form>

I have little to no experience when it comes to client side programming so I have no idea how to implement this; I tried looking for a bunch of JS/jQuery codes but none seem to work:/

Any help is appreciated!

Answer 1

Add a change event listener to the form to handle changing the required attribute of the input element beside the select element whose value changed.

document.querySelector('form').addEventListener('change', e => {
    if (e.target.matches('select')) {
        e.target.previousElementSibling.required = e.target.value !== 'j.e.';
    }
});

Answer 2

If I understand correctly, This is my working solution:

I set id for your Qt inputs. And I removed one with jQuery.

<form method="post">
<?php
    for ($n = 0; $n < $num; $n++) {
        $data = $DB->query("SELECT * FROM ingredient"); ?>
        <select name="list_ingr[]" required> <?php
        while ($ingrs = $data->fetch()) { ?>
            <option value="<?php echo $ingrs['id_ingr'] ?>"><?php echo $ingrs['name'] ?></option> - <?php
        } ?>
        </select>

        Qt. <input id="qt_<?php echo $ingrs['id_ingr'] ?>" type="number" name="list_quant[]" step="1" min="1" required>
        <select name="measure[]">
            <option value="none"></option>
            <option value="gr">gr</option>
            <option value="ml">ml</option>
            <option value="j.e.">j.e.</option>
        </select><br>
    <?php    
    }
?>
<br>
<input type="submit" name="confirm" value="Confirm">
</form>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.6.1/jquery.min.js"></script>
<script>
$(document).ready(function(){
    $('#qt_2').remove(); //For example, to remove qt_2
});
</script>