Web Components - Select

An implementation of an HTML select element as a form-connected Web Component. It is very similar to the combo-box, but with some differences such as:

• Users can only select from the list provided.
• There is no auto-completion.
• Users can select multiple options.
• The value of the selected option can be different from its label.

Set-up

import { provideDesignSystem, alphaSelect, alphaOption } from '@genesislcap/alpha-design-system';

provideDesignSystem().register(alphaSelect(), alphaOption());

Attributes

This component needs to be used in combination with <alpha-option>. You can define the following attributes when you declare an <alpha-select>:

Name	Type	Description
disabled	boolean	Similar to readonly, but with a blur on the component
form	string	Associates this component with a form. Form id needs to be passed. If no Id is provided, then it will be associated with the ancestor form
multiple	boolean	Enables the user to select more than one option; this automatically opens the selection and removes the side arrow. Default: false
name	string	Gives this component a name
open	boolean	Defines whether the list starts opened or not. Default: false
position	string	Places the list above or below the component; can be above or below. Default: it will try to fit with the page
size	number	Defines the maximum number of options to be displayed. Default: it will try to fit with the page
value	string	Sets a value for this component

Option attributes

In order to use the select component, you need to create a list of options for the user to select from. You create this list using <alpha-option>. You can define the following attributes for an <alpha-option>:

Name	Type	Description
disabled	boolean	Disables an option so that the user cannot interact with it, but the option is still visible
selected	boolean	Selects the option, so it turns to the selected mode

note

• If you specify a selected or value to more than one option while multiple = false, then the component selects only the first in the alpha-option list

Usage

All examples below use the alpha-design-system. If you are using any other design system, change the declaration of this component accordingly.

• Example 1: a select displaying 2 options at a time; users can scroll up or down to see other options in the list

Example 1

<alpha-select size="2" open>
    <alpha-option value="s">Small</alpha-option>
    <alpha-option value="m">Medium</alpha-option>
    <alpha-option value="l">Large</alpha-option>
    <alpha-option value="xl">Extra Large</alpha-option>
</alpha-select>

• Example 2: a disabled select greyed out so that users can not select an option; 2 options selected: "s" and "l"

Example 2

<alpha-select disabled multiple>
    <alpha-option selected value="s">Small</alpha-option>
    <alpha-option value="m">Medium</alpha-option>
    <alpha-option selected value="l">Large</alpha-option>
    <alpha-option value="xl">Extra Large</alpha-option>
</alpha-select>

• Example 3: a select with a starting value of xl; when the user clicks, the list is displayed above the select

Example 3

<alpha-select position="above" value="xl">
    <alpha-option value="s">Small</alpha-option>
    <alpha-option value="m">Medium</alpha-option>
    <alpha-option value="l">Large</alpha-option>
    <alpha-option value="xl">Extra Large</alpha-option>
</alpha-select>

Get the user input

Once the user has input a value to this component, you need to make it accessible to the application:

1. Create an @observable variable where you want to use this value:

import {... , observable} from '@microsoft/fast-element';
...
export class TEMPLATE extends FASTElement {
...
@observable select_value: string
...
}

2. Use the sync function to insert the value from the component into the variable select_value:

import {sync} from '@genesislcap/foundation-utils';
...
    ...
        <alpha-select :value=${sync((x) => x.select_value)}>
            <alpha-option value="1">Small</alpha-option>
            <alpha-option value="2">Medium</alpha-option>
            <alpha-option value="3">Large</alpha-option>
            <alpha-option value="4">Extra Large</alpha-option>
        </alpha-select>
    ...
...    

From this point, you can access the value of the component in your application.

Create a list dynamically

When you're developing your application, you normally want to generate the list instead of creating each option manually.

To do this, follow the steps below:

1. Create an @observable Array<{value: string, label: string}> with the options to be available in your select:

import {... , observable} from '@microsoft/fast-element';
...
export class TEMPLATE extends FASTElement {
    ...
    @observable options: Array<{value: string, label: string}> = []
    ...
}

Note that we are creating a complex array with two attributes: value and label. That way, it is easier to assign it in the repeat directive.

With this variable created, you can generate any array you wish using a loop. Here is a simple example:

    for (let i = 0; i < 4; i++) {
      this.options.push({value:"v" + i, label: "Label " + i});
    }

2. Use the repeat directive to create an <alpha-option> for each element you have in options

import {... , repeat} from '@microsoft/fast-element';
...
    ...
        <alpha-select>
            ${repeat(x => x.options, html` <alpha-option value=${x => x.value}>${x => x.label}</alpha-option>`)}
        </alpha-select>
    ...
...    

The repeat directive takes two arguments:

• The first one is the array/object to be iterated. In the code above, we have x => x.options because the variable we created is an array named options.
• The second is the component to be repeated for each element in the array/object passed in the first argument. In this case, we are repeating the component alpha-option.

If you are not familiar with the repeat directive, take a look at the Microsoft Fast documentation.

Try yourself

try yourself

<alpha-select>
  <alpha-option value="s">Small</alpha-option>
  <alpha-option value="m">Medium</alpha-option>
  <alpha-option value="l">Large</alpha-option>
  <alpha-option value="xl">Extra Large</alpha-option>
</alpha-select>

Use cases

• Selecting from a list of options
• Selecting multiple options
• Sorting
• Filtering

Additional resources

• W3C Component Aria Practices