Web Component for interdependent <select> form elements, where the available options in a dependent selection list are based on the selection in a parent or control list. This pattern is also known as "dependent select" (a subform of the generic "input-dependent input") or "cascading dropdowns".

Installation

npm install @webfactoryde/cascading-select

Usage

The <cascading-select> Web Component is a lightweight wrapper for standard form markup. If offers complete flexibility in regard to the use of <fieldset>, <legend>, <label> and other elements or attributes (i. e. class) needed for layout and styling, as long as two <select> elements with the necessary (data-) attributes are present.

The second, dependent <select> should initially be hidden (via a hidden attribute). This way, in case there are JavaScript errors, the parent <select> does still offer high-level filtering and only the progressive enhancement of the secondary filter level is missing.

Steps to implement:

1. The JS file "cascading-select.js" must be loaded. Depending on browser support requirements, transpilation for older browsers is recommended.
2. Two <select> elements must be output within the Web Component.
3. The parent <select> requires a data-dependent-id attribute that contains the id of the dependent <select> as its value to link the two fields.
4. The <option>s of the parent <select> that have a sub-selection at the second level require a data-dependent-options attribute with a JSON-formatted array of objects, each containing a label and value. Empty placeholder options must also be listed.
5. The dependent <select> requires an id attribute with matches the value used for data-dependent-id on the parent.
6. The dependent <select> should initially have a hidden attribute; this will be removed by the Web Component as needed, but serves as a no-JS fallback in case the Web Component does not initialize correctly.

Parameters

Web Components support common HTML attributes but can also receive custom parameters.

Option	Type	Default	Description
always-show-dependent	boolean	false	Controls initial display of the second ("dependent") <select> when no selection is made in the first ("parent") or an option without sub-selection possibilities is selected. Default: the second field is hidden. When the parameter is present, it is displayed and marked with aria-disabled (assuming it has no meaningful interactivity until a selection is made in the "parent", but should still be visible to visually indicate the functional pattern).

Example

<cascading-select>
    <fieldset>
        <legend>Where do you want to go bouldering?</legend>
        <div>
            <label for="town">City</label>
            <select id="town" name="town" data-dependent-id="gym">
                <option value="">Please choose a city</option>
                <option
                    value="1"
                    data-dependent-options='[
                        { "value": "", "label": "Please choose a gym" },
                        { "value": "d11", "label": "Boulders Habitat Bonn" },
                        { "value": "d12", "label": "Boulders Habitat Beuel" }
                    ]'
                >
                    Bonn
                </option>
                <option
                    value="2"
                    data-dependent-options='[
                        { "value": "", "label": "Please choose a gym" },
                        { "value": "d21", "label": "Boulderplanet" },
                        { "value": "d22", "label": "Stuntwerk Köln" },
                        { "value": "d23", "label": "Kletterfabrik" },
                        { "value": "d24", "label": "K11" }
                    ]'
                >
                    Köln
                </option>
                <option
                    value="3"
                    data-dependent-options='[
                        { "value": "", "label": "Please choose a gym" },
                        { "value": "d31", "label": "Monolith" },
                        { "value": "d32", "label": "Boulderfactory" },
                        { "value": "d33", "label": "KletterBar" }
                    ]'
                >
                    Münster
                </option>
            </select>
        </div>
        <div>
            <label for="gym">Gym</label>
            <select id="gym" class="form-control"></select>
        </div>
    </fieldset>
</cascading-select>