Behavior of .fill with multiselect model when initial property value is [] vs null vs ''

[epicwhale]

As per the documentation of .fill: "if a bound property is empty, then you can use an input's value attribute to populate the property by adding the .fill modifier"

While this works for input fields whose value is a null or empty '' string, when I use this with a multiple select field, and if the initialised property value is an empty [] array, it does not seem to replace it with the select's values.

For example:

This sets the selected_values state property to [2,3]

<form x-data="{ selected_values: null }">
            <select multiple x-model.fill="selected_values">
                <option value="1">One</option>
                <option value="2" selected>Two</option>
                <option value="3" selected>Three</option>
            </select>
            <div x-text="selected_values"></div>
        </form>

This does not work. It sets the selected_values state property to [] instead of [1.2]

<form x-data="{ selected_values: [] }">       <!-- note i've made the null to [] -->
            <select multiple x-model.fill="selected_values">
                <option value="1">One</option>
                <option value="2" selected>Two</option>
                <option value="3" selected>Three</option>
            </select>
            <div x-text="selected_values"></div>
        </form>

On closer inspection, it looks like the culprit could be this statement which only checks for null and === '', but not an empty array:

alpine/packages/alpinejs/src/directives/x-model.js

Line 50 in 2538570

if (modifiers.includes('fill') && el.hasAttribute('value') && (getValue() === null || getValue() === '')) {

I was wondering if this behavior is intended?

[ekwoka]

The check itself is as intended.

There is a separate check if you look for checkboxes filling to arrays.

As you know, a Select multiple has fundamentally different behaviors compared to other inputs and may need it's own special check.

I'd probably consider the fact it doesn't work with select multiple to be a bug though.

[epicwhale]

You're right, I more meant there perhaps needs to be a more fine-tuned .fill behavior tailored for multiple select, like there's one for input fields today! Unsure, how something makes it from a discussion to bug here, beginner here :) so defer it to the folks here!

[ekwoka]

There is no issues.

You can make a PR to get it checked out and maybe merged

[bb]

Should be fixed with #4159. Please give it a try.