Custom inputs
FormKit includes many inputs out of the box, but you can also define your own inputs that automatically inherit FormKit’s value-added features like validation, error messages, data modeling, grouping, labels, help text and others.
If your use case requires modifications of an existing input, such as moving sections, changing or restructuring HTML elements, etc., consider using FormKit's input export feature.
Inputs are comprised of two essential parts:
- An input definition.
- The input’s code: a schema or a component.
If you are just getting started with custom inputs, consider reading the “Create a custom input” guide. The content on this page is intended to explain the intricacies of custom inputs for advanced use cases like authoring a plugin or library and is not required for many common use cases.
Registering inputs
New inputs require an input definition. Input definitions can be registered with FormKit three ways:
- Locally on a
FormKitcomponent with thetypeprop. - Globally using defaultConfig.
- Selectively using plugin libraries.
Input definition
Input definitions are objects that contain the necessary information to initialize an input — like which props to accept, what schema or component to render, and if any additional feature functions should be included. The shape of the definition object is:
{
// Node type: input, group, or list.
type: 'input',
// Schema to render (schema object or function that returns an object)
schema: [],
// A Vue component to render (use schema _OR_ component, but not both)
component: YourComponent,
// (optional) Input specific props the <FormKit> component should accept.
// should be an array of camelCase strings
props: ['fooBar'],
// (optional) Array of functions that receive the node.
features: []
}
Using the type prop
Let’s make the simplest possible input — one that only outputs "Hello world".
<script setup>const helloWorld = { type: 'input', schema: ['Hello world'],}</script><template> <FormKit :type="helloWorld" /></template>Even though this simplistic example doesn’t contain any input/output mechanism, it still qualifies as a full input. It can have a value, run validation rules (they wont be displayed, but they can block form submissions), and execute plugins. Fundamentally, all inputs are core nodes and the input’s definition provides the mechanisms to interact with that node.
Global custom inputs
To use your custom input anywhere in your application via a "type" string (ex: <FormKit type="foobar" />) you can add an inputs property to the defaultConfig options. The property names of the inputs object become the "type" strings available to the <FormKit> component in your application.
import { createApp } from 'vue'
import App from 'App.vue'
import { plugin, defaultConfig } from '@formkit/vue'
const helloWorld = {
type: 'input',
schema: ['Hello world'],
}
createApp(App)
.use(
plugin,
defaultConfig({
inputs: {
// The property will be the “type” in <FormKit type="hello">
hello: helloWorld,
},
})
)
.mount('#app')
Now that we’ve defined our input we can use it anywhere in the application:
<template> <FormKit type="hello" /></template>Plugin libraries
The above example extends the @formkit/inputs library (via defaultConfig). However, a powerful feature of FormKit is its ability to load input libraries from multiple plugins. These inputs can then be registered anywhere plugins can be defined:
- Globally
- Per group
- Per form
- Per list
- Per input
Let’s refactor our hello world input to use its own plugin:
<script setup>// We’ll use the same input definition.const helloDefinition = { type: 'input', schema: ['Hello world'],}// In this case our plugin’s body doesn't actually do anythingconst myPlugin = () => {}// Then we attach a librarymyPlugin.library = (node) => { if (node.props.type === 'hello') { node.define(helloDefinition) }}</script><template> <FormKit type="form" :plugins="[myPlugin]" @submit="() => false"> <FormKit type="hello" /> </FormKit></template>Notice in the above example our plugin was defined on a parent of the element that actually used it! This is thanks to plugin inheritance — a core feature of FormKit plugins.
Schema vs component
Your input can be written using FormKit’s schema or a generic Vue component. There are pros and cons to each approach:
| Code | Pros | Cons |
|---|---|---|
| Vue |
|
|
| Schema |
|
|
Even if you prefer to write a custom input using a standard Vue Component, you can still use a schema in your input definition. Please read the Using createInput to extend the base schema section.
The primary takeaway is if you are planning to use a custom input on multiple projects — then consider using the schema-based approach. If your custom input will only be used on a single project and flexibility is not a concern, use a Vue component.
Future proofing
In the future, FormKit may expand to support additional frameworks (ex: React or Svelte. If this is something you are interested in, let us know!.) Writing your inputs using schema means your inputs will be compatible (perhaps minimal changes) with those frameworks too.
Schema inputs
All of FormKit’s core inputs are written using schemas to allow for the greatest flexibility possible. You have two primary options when writing your own schema inputs:
- Extend the base schema (recommended).
- Write your input from scratch.
It is important to understand the basic structure of a “standard” FormKit input, which is broken down into sections:
Click on a section to lock focus to the chosen section. Click locked section again to unlock.
The input section in the diagram above is typically what you’ll swap out when creating your own inputs — keeping the wrappers, labels, help text, and messages intact. However, if you want to control these aspects as well, you can also write your own input from scratch.
Using createInput to extend the base schema
To create inputs using the base schema you can use the createInput() utility from the @formkit/vue package. This function accepts 3 arguments:
- (required) A schema node or a Vue component, which it inserts into the base schema at the
inputsection (see diagram above). - (optional) An object of input definition properties to merge with an auto-generated one.
- (optional) A sections-schema object (just like the sections-schema prop) to merge with the base schema. This lets you modify the wrapping structure of the input.
The function returns a ready-to-use input definition.
When providing a component as the first argument, createInput will generate a schema object that references your component within the base schema. Your component will be passed a single context prop:
{
$cmp: 'YourComponent',
props: {
context: '$node.context'
}
}
When providing a schema object, your schema is directly injected into the base schema object. Notice that our hello world example now supports outputting "standard" FormKit features like labels, help text, and validation:
<script setup>import { createInput } from '@formkit/vue'const myInput = createInput('Hello world')</script><template> <FormKit :type="myInput" label="Hello world example" help="You still cant interact with me, but im here!" validation="required" validation-visibility="live" /></template>- Hello world example is required.
Writing schema inputs from scratch
It might make sense to write your inputs completely from scratch without using any of the base schema features. When doing so, just provide the input definition your full schema object.
<script setup>const myInput = { type: 'input', schema: [ { $el: 'label', if: '$label', attrs: { class: '$classes.label', }, children: '$label', }, { $el: 'div', attrs: { class: '$classes.inner', }, children: ['Hello world'], }, { $el: 'div', if: '$help', attrs: { class: '$classes.help', }, children: '$help', }, { $el: 'ul', attrs: { class: '$classes.messages', }, children: [ { $el: 'li', for: ['message', '$messages'], attrs: { class: '$classes.message', }, children: '$message.value', }, ], }, ],}</script><template> <FormKit :type="myInput" label="Hello world example" help="You still cant interact with me, but im here!" validation="required" validation-visibility="live" /></template>- Hello world example is required.
In the above example, we were able to re-create the same features as the createInput example — namely — label, help text, and validation message output. However, we are still missing a number of "standard" FormKit features like slot support. If you are attempting to publish your input or maintain API compatibility with the other FormKit inputs, take a look at the input checklist.
Component inputs
When writing a custom FormKit input while using Vue components it is recommended to not use the FormKit components inside, custom inputs are meant to be written like regular inputs with the advantage of using the FormKit context prop to add the functionality that FormKit requires, if your case is to use a FormKit component with default values, it is recommended instead to use a Vue component wrapper and directly call that component, FormKit inputs work in any level of nesting, or you can also consider using FormKit's input export feature to add features and change attrs and props.
For most users, passing a Vue component to createInput provides a good balance between customization and value-added features. If you’d like to completely eject from schema-based inputs all together, you can pass a component directly to an input definition.
Component inputs receive a single prop — the context object. It’s then up to you to write a component to encompasses the desired features of FormKit (labels, help text, message display, etc.). Checkout the input checklist for a list of what you’ll want to output.
Input & output values
Inputs have two critical roles:
- Receiving user input.
- Displaying the current value.
Receiving input
You can receive input from any user interaction and the input can set its value to any type of data. Inputs are not limited to strings and numbers — they can happily store Arrays, Objects, or custom data structures.
Fundamentally, all an input needs to do is call node.input(value) with a value. The node.input() method is automatically debounced, so feel free to call it frequently — like every keystroke. Typically, this looks like binding to the input event.
The context object includes an input handler for basic input types: context.handlers.DOMInput. This can be used for text-like inputs where the value of the input is available at event.target.value. If you need a more complex event handler, you can expose it using "features".
Any user interaction can be considered an input event. For many native HTML inputs, that interaction is captured with the input event.
// An HTML text input written in schema:
{
$el: 'input',
attrs: {
onInput: '$handlers.DOMInput'
}
}
The equivalent in a Vue template:
<template>
<input @input="context.DOMInput" />
</template>
Displaying values
Inputs are also responsible for displaying the current value. Typically, you’ll want to use the node._value or $_value in schema to display a value. This is the "live" non-debounced value. The currently committed value is node.value ($value). Read more about "value settlement" here.
// An HTML text input written in schema:
{
$el: 'input',
attrs: {
onInput: '$handlers.DOMInput',
value: '$_value'
}
}
The equivalent in a Vue template:
<template>
<input :value="context._value" @input="context.handlers.DOMInput" />
</template>
The only time the uncommitted input _value should be used is for displaying the value on the input itself — in all other locations, it is important to use the committed value.
Adding props
The standard FormKit props that you can pass to the <FormKit> component (like label or type) are available in the root of the context object and in the core node props, and you can use these props in your schema by directly referencing them in expressions (ex: $label). Any props passed to a <FormKit> component that are not node props end up in the context.attrs object (just $attrs in the schema).
If you need additional props, you can declare them in your input definition. Props can also be used to accept new props from the <FormKit> component but they are also used for internal input state (much like a ref in a Vue 3 component).
FormKit uses the props namespace for both purposes (see the autocomplete example below for an example of this). Props should always be defined in camelCase and used in your Vue templates with kebab-case. There are 2 ways to define props:
Array notation
<script setup>// This is our input definition:const hello = { type: 'input', // Look! A new prop is born: props: ['location'], // We can use the prop directly in our schema: schema: [ { $el: 'h1', children: ['Hello ', '$location'], }, ],}</script><template> <FormKit :type="hello" location="Mars" /></template>Hello Mars
When extending the base schema by using the createInput helper, pass a second argument with input definition values to merge:
<script setup>import { createInput } from '@formkit/vue'const myInput = createInput( { $el: 'input', attrs: { class: '$classes.input', placeholder: '$: "Hello " + $location', }, }, { props: ['location'], family: ['text'], // inherit theme classes from text family })</script><template> <FormKit :type="myInput" label="Hello where?" location="Spain" help="We use the custom prop location in our placeholder." /></template>Object notation
Object notation gives you fine grained control over how your props are defined by giving you the ability to:
- Define a default value.
- Define
booleanprops that can be passed without a value. - Define custom getter/setter functions.
<script setup>import { createInput } from '@formkit/vue'const myInput = createInput( { $el: 'button', for: ['option', '$options'], attrs: { class: '$classes.input', placeholder: '$: "Hello " + $location', onClick: '$handlers.setValue($option)', ariaSelected: '$: $option === $value', }, children: ['$currency', '$option'], }, { props: { // 👀 A boolean prop — these can be added as attributes to the component capitalize: { boolean: true, }, // 👀 Here we have a prop with a default value currency: { default: '', }, // 👀 This prop has a setter that transforms the prop value anytime it is set options: { setter(value, node) { let options if (typeof value === 'string') { options = value.split(',') } else { options = value } if (node.props.capitalize) { options = options.map( (option) => option[0].toUpperCase() + option.slice(1) ) } return options }, }, }, features: [ function setValues(node) { node.on('created', () => { // Notice the currying here bind the value into the callback node.context.handlers.setValue = (value) => () => node.input(value) }) }, ], })</script><template> <FormKit type="group" #default="{ value }"> <FormKit :type="myInput" name="planet" label="Select a hospitable planet" capitalize options="earth,mars,venus" input-class="border-2 border-gray-300 mr-2 mb-2 px-2 py-1 rounded-md shadow-sm focus:ring-indigo-500 focus:border-indigo-500 sm:text-sm aria-[selected=true]:bg-gray-200 dark:bg-gray-800 dark:aria-[selected=true]:bg-blue-900 dark:border-gray-700 dark:focus:ring-gray-500 dark:focus:border-gray-500" /> <FormKit :type="myInput" name="price" label="Price" currency="$" :options="['10.00', '12.99', '15.00']" help="How much are you willing to pay?" input-class="border-2 border-gray-300 mr-2 mb-2 px-2 py-1 rounded-md shadow-sm focus:ring-indigo-500 focus:border-indigo-500 sm:text-sm aria-[selected=true]:bg-gray-200 dark:bg-gray-800 dark:aria-[selected=true]:bg-blue-900 dark:border-gray-700 dark:focus:ring-gray-500 dark:focus:border-gray-500" /> <pre wrap>{{ value }}</pre> </FormKit></template>{}Add props method (node.addProps())
You can dynamically add props using the node.addProps() method in any runtime environment where you have access to the node. For custom inputs this is particularly helpful when used in a features. Both array notation and object notation are supported (see above).
<script setup>import { createInput } from '@formkit/vue'function useLocation(node) { node.addProps({ location: { default: 'World', }, })}const myInput = createInput( { $el: 'input', attrs: { class: '$classes.input', placeholder: '$: "Hello " + $location', }, }, { family: ['text'], // inherit theme classes from text family features: [useLocation], })</script><template> <FormKit :type="myInput" label="Hello where?" location="Andromeda" help="We used a feature with node.addProps() to add a location" /></template>Adding features
Features are the preferred way to add functionality to a custom input type. A "feature" is a function that receives the core node as an argument. Effectively, they are plugins without inheritance (so they only apply to the current node). You can use features to add input handlers, manipulate values, interact with props, listen to events, and much more.
Features are defined in an array to encourage code reuse when possible. For example, we use a feature called “options” on select, checkbox, and radio inputs.
As an example, let's imagine you want to build an input that allows users to enter two numbers, and the value of the input is the sum of those two numbers:
<script setup>import { ref } from 'vue'import { createInput } from '@formkit/vue'// Here's our schema, notice we have 2 handlers:// numberA and numberB. Those are not provided by FormKit by default, they are// defined in our "feature".const sum = createInput( [ { $el: 'input', attrs: { class: '$classes.addend', // set in formkit.config.ts type: 'number', onInput: '$handlers.numberA', }, }, '+', { $el: 'input', attrs: { class: '$classes.addend', // set in formkit.config.ts type: 'number', onInput: '$handlers.numberB', }, }, '= ', '$_value || 0', ], { features: [addHandlers], family: 'text', // inherit theme classes from text family })/** * Here we add our handlers. */function addHandlers(node) { // Features are run very early in the node’s boot — before the context object // is fully initialized, so we wait till the node is "created" — this is still // before the node is mounted, but after plugins have run. let numberA = 0 let numberB = 0 node.on('created', () => { // Now we define a new handler. Note that context.handlers is the same // object available in $handlers in our schema. node.context.handlers.numberA = (event) => { numberA = Number(event.target.value) // We update the value of the input by calling node.input() node.input(numberA + numberB) } // Now a second context handler node.context.handlers.numberB = (event) => { numberB = Number(event.target.value) node.input(numberA + numberB) } })}const groupValues = ref({})</script><template> <FormKit type="group" v-model="groupValues"> <p class="mb-4"> To illustrate the final value of this input, let's stick it in a group. </p> <FormKit :type="sum" name="mySum" label="Sum 2 numbers" help="Enter two numbers above and their values will be summed!" /> </FormKit> <pre wrap>{{ groupValues }}</pre></template>To illustrate the final value of this input, let's stick it in a group.
{}TypeScript support
FormKit is written in TypeScript and includes type definitions for all of its core inputs. If you are writing your own inputs and would like to provide TypeScript support you can define your own inputs using two module augmentations:
Adding prop types
The type prop of the <FormKit> component is a string that is used as the key of a discriminated union of props (FormKitInputProps). By augmenting this type your custom inputs can define their own prop types. To do so you must augment the FormKitInputProps type to add your own custom types:
declare module '@formkit/inputs' {
interface FormKitInputProps<Props extends FormKitInputs<Props>> {
// This key and the `type` should match:
'my-input': {
// Define your input `type`:
type: 'my-input',
// Define an optional prop. Use camelCase for all prop names:
myOptionalProp?: string | number
// Define a required prop
superImportantProp: number
// Define the value type, this should always be a optional!
value?: string | number
// Use the Prop generic to infer information from another field, notice
// we a utility "PropType" to infer the type of the `value` from the Props
// generic:
someOtherProp?: PropType<Props, 'value'>
}
}
}
Adding slot types
If you define your own sections (slots) in your custom input, you can also add TypeScript support for those too. To do so, you must augment the FormKitInputSlots type to add your own custom slots:
declare module '@formkit/inputs' {
interface FormKitInputProps<Props extends FormKitInputs<Props>> {
'my-input' {
type: 'my-input'
// ... props here
}
}
interface FormKitInputSlots<Props extends FormKitInputs<Props>> {
'my-input': FormKitBaseSlots<Props>
}
}
In the example above, we use FormKitBaseSlots — a TypeScript utility to add all the "basic" slots that most custom inputs implement, like outer, label, help, message, etc. However you could also define your own slots entirely from scratch, or augment FormKitBaseSlots to add additional slots (FormKitBaseSlots<Props> & YourCustomSlots).
declare module '@formkit/inputs' {
// ... props here
interface FormKitInputSlots<Props extends FormKitInputs<Props>> {
'my-input': {
// This will be the *only* slot available on the my-input input
slotName: FormKitFrameworkContext & {
// this will be available as slot data in the `slotName` slot
fooBar: string
}
}
}
}
}
In order to augment the FormKitInputSlots, you must first have written an augmentation for FormKitInputProps that at least includes the type prop.
Examples
Below are some examples of custom inputs. They are not intended to be comprehensive or production ready, but rather illustrate some custom input features.
Simple text input
This is the simplest possible input and does not leverage any of FormKit’s built in DOM structure and only outputs a text input — however it is a fully functional member of the group it is nested inside of and able to read and write values.
<script setup>import { ref } from 'vue'const groupValues = ref({})const simpleText = { type: 'input', schema: [ { $el: 'input', attrs: { onInput: '$handlers.DOMInput', value: '$_value', class: 'border border-gray-900 bg-transparent dark:border-gray-500', }, }, ],}</script><template> <FormKit type="group" v-model="groupValues"> <FormKit :type="simpleText" name="simple" /> </FormKit> <pre wrap>{{ groupValues }}</pre></template>{}In the above example the $handlers.DOMInput is a built-in convenience function for (event) => node.input(event.target.value).
Autocomplete input
Let’s take a look at a slightly more complex example that utilizes createInput to provide all the standard FormKit structure while still providing a custom input interface.
<template> <FormKit label="U.S. State" type="myAutocomplete" placeholder="Search for a state" help="What is your favorite U.S. state?" value="Virginia" :options="stateList" /></template>import { createInput } from '@formkit/vue'/** * This is an input "feature" — a function that accepts a node and exposes * some additional functionality to an input. When using schemas, this can * take the place of a traditional "script" block in a Vue component. In this * example, we expose: * * - An input handler `search`. * - An input handler `selections`. * - Commit middleware to place filtered options into the `matches` prop. * * Once written, input features are added via the input declaration. */const searchFeature = (node) => { // We wait for our node to be fully "created" before we start to add our // handlers to ensure the core Vue plugin has added its context object: node.on('created', () => { // Ensure our matches prop starts as an array. node.props.matches = [] node.props.selection = '' // When we actually have an value to set: const setValue = async (e) => { if (e && typeof e.preventDefault === 'function') e.preventDefault() node.input(node.props.selection) node.props.selection = '' node.props.searchValue = '' await new Promise((r) => setTimeout(r, 50)) // "next tick" if (document.querySelector('input#' + node.props.id)) { document.querySelector('input#' + node.props.id).focus() } } // Perform a soft selection, this is shown as a highlight in the dropdown const select = (delta) => { const available = node.props.matches let idx = available.indexOf(node.props.selection) + delta if (idx >= available.length) { idx = 0 } else if (idx < 0) { idx = available.length - 1 } node.props.selection = available[idx] } // Add some new "handlers" for our autocomplete. The handlers object is // just a conventionally good place to put event handlers. Auto complete // inputs always have to deal with lots of keyboard events, so that logic // is registered here. Object.assign(node.context.handlers, { setValue, selection: (e) => { // This handler is called when entering data into the search input. switch (e.key) { case 'Enter': return setValue() case 'ArrowDown': e.preventDefault() return select(1) case 'ArrowUp': e.preventDefault() return select(-1) } }, search(e) { node.props.searchValue = e.target.value }, hover: (e) => { node.props.selection = e.target.textContent }, unhover: (e) => { if (e.target.textContent === node.props.selection) { node.props.selection = '' } }, }) }) // Perform filtering when the search value changes node.on('prop:searchValue', ({ payload: value }) => { const results = node.props.options.filter((option) => option.toLowerCase().startsWith(value.toLowerCase()) ) if (!results.length) results.push('No matches') node.props.matches = results })}/** * This is our input schema responsible for rendering the inner “input” * section. In our example, we render an text input which will be used * to filter search results, and an unordered list that shows all remaining * matches. */const schema = { if: '$value', then: [ { $el: 'a', attrs: { id: '$id', href: '#', class: '$classes.value', onClick: '$handlers.setValue', }, children: '$value', }, ], else: [ { $el: 'input', bind: '$attrs', attrs: { id: '$id', class: '$classes.input', onKeydown: '$handlers.selection', onInput: '$handlers.search', value: '$searchValue', }, }, { $el: 'ul', if: '$matches.length < $options.length', attrs: { class: '$classes.dropdown', }, children: [ { $el: 'li', for: ['match', '$matches'], attrs: { 'data-selected': { if: '$selection === $match', then: 'true', else: 'false', }, class: '$classes.dropdownItem', onClick: '$handlers.setValue', onMouseenter: '$handlers.hover', onMouseleave: '$handlers.unhover', }, children: '$match', }, ], }, ],}/** * Finally we create our actual input declaration by using `createInput` this * places our schema into a "standard" FormKit schema feature set with slots, * labels, help, messages etc. The return value of this function is a proper * input declaration. */const autocomplete = createInput(schema, { props: ['options', 'matches', 'selection', 'searchValue'], features: [searchFeature], family: 'text', // choose which family to inherit base class lists from})export default autocompleteInput checklist
FormKit exposes dozens of value-added features to even the most mundane inputs. When writing a custom input for a specific project, you only need to implement the features that will actually be used on that project. However, if you plan to distribute your inputs to others, you will want to ensure these features are available. For example, the standard <FormKit type="text"> input uses the following schema for its input element:
{
$el: 'input',
bind: '$attrs',
attrs: {
type: '$type',
disabled: '$disabled',
class: '$classes.input',
name: '$node.name',
onInput: '$handlers.DOMInput',
onBlur: '$handlers.blur',
value: '$_value',
id: '$id',
}
}
There are several features in the above schema that may not be immediately obvious like the onBlur handler. The following checklist is intended to help input authors cover all their bases: