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@RocketPuppy RocketPuppy/Form.tsx Secret
Last active Jan 30, 2019

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Typescript Generic Form Component
// A higher order component for modeling simple stateful forms. Type-checked with Typescript.
import * as R from "ramda";
import * as React from "react";
import * as ReactDOM from "react-dom";
// Props for the HOC, generic over the form fields specified by the passed in component.
interface Props<FormFields> {
FormComponent: React.ComponentType<FormComponentProps<FormFields>>;
initialValues: FormFields;
// The Props that the inner component will receive. They include all the fields specified by FormFields and the Handlers.
type FormComponentProps<FormFields> = { fields: FormFields } & Handlers<
// The on change handler type. This is extracted because it's too big to put inline in the Form class. Some
// cool stuff going on here. The `<K extends keyof FormFields>` makes the function generic over all keys of
// FormFields. Furthermore, the first argument must be a key that exists in FormFields, and the second
// argument must be the type of the value specified for that key. You can't ever misspell a key by accident,
// or pass an incorrect type for the value here.
type OnChangeHandler<FormFields> = <K extends keyof FormFields>(
s: K,
a: FormFields[K]
) => void;
// This just specifies the Handlers. Parameterized over FormFields like everything else.
interface Handlers<FormFields> {
onChange: OnChangeHandler<FormFields>;
// The state of the Form component is just FormFields but wrapped in another object. This wrapping makes it
// easier to type-check the setState calls.
interface State<FormFields> {
fields: FormFields;
// Types are basically over, now for the meat of the class itself. First it's a generic class, which means
// when you use it you need to specialize it by filling in the type for FormFields. I'll explain how this
// works in detail later. Because it has a generic parameter that generic parameter is available throughout
// the body of the class so you can use it in types there. The actual implementation of everything is pretty
// straightforward.
class Form<FormFields> extends React.Component<
> {
constructor(props: Props<FormFields>) {
this.state = { fields: props.initialValues };
public onChange: OnChangeHandler<FormFields> = (field, value) => {
this.setState({ fields: R.merge(this.state.fields, { [field]: value }) });
public render() {
const { FormComponent } = this.props;
const { fields } = this.state;
return <FormComponent onChange={this.onChange} fields={fields} />;
export default Form;
// So down to usage. If you want to use this in a JSX expression you need to specialize it first. There's no
// syntax in typescript to do this inline unfortunately but the actual method isn't too onerous. Basically
// all you need to do is declare a new class that extends the Form class, specifying a type parameter for it.
// First specify the form fields you want.
interface SportsFields {
teamName: string;
playerNumber: number;
class SportsForm extends Form<SportsFields> {};
// One thing you'll notice here is there's no need to add type annotations to any of the props. In fact I've
// noticed that if you do add type annotations it confuses Typescript.
initialValues={{ teamName: [], playerNumber: 0 }}
FormComponent={({ fields: { teamName, playerNumber }, onChange }) => (
<input type='text' value={teamName} onChange={(e) => onChange("teamName",} />
<input type='number' value={playerNumber} onChange={(e) => onChange("playerNumber", parseInt(, 10))} />
// And that's it! Try to mispell those field names. Typescript won't let you, and you also can't mix up the
// values passed to the onChange handler for each field name.

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subvertallchris commented Feb 5, 2018

This is very cool. I hadn't thought of defining a class that extends Form<SportsFields> to feed my generics into the form but it's perfect. Thanks for demonstrating.

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