Custom inputs

• Registering inputs
  • Input definition
  • Using the type prop
  • Global custom inputs
  • Plugin libraries
• Schema vs component
  • Future proofing
• Schema inputs
• Component inputs
• Input & output values
  • Receiving input
  • Displaying values
• Adding props
  • Array notation
  • Object notation
  • Add props method (node.addProps())
• Adding features
• TypeScript support
  • Adding prop types
  • Adding slot types
• Examples
  • Simple text input
  • Autocomplete input
• Input checklist

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.

Inputs are comprised of two essential parts:

1. An input definition.
2. The input’s code: a schema or a component.

Registering inputs

New inputs require an input definition. Input definitions can be registered with FormKit three ways:

• Locally on a FormKit component with the type prop.
• 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 any additional feature functions that 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".

Even though this simple example doesn’t contain any input/output mechanism, it still qualifies as a full input. It can have a value, run validation rules (they won’t 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:

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:

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:

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 with 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:

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 input section (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:

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.

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

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 that 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>

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:

1. Array notation.
2. Object notation.
3. The node.addProps() method

Array notation

When extending the base schema by using the createInput helper, pass a second argument with input definition values to merge:

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 boolean props that can be passed without a value.
• Define custom getter/setter functions.

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 array. Both array notation and object notation are supported (see above).

Adding features

Features are the preferred way to add functionality to a custom input type. A "feature" is simply 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:

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
        }
      }
    }
  }
}

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. It 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.

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.

Input 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: