[javascript] typesafe select onChange event using reactjs and typescript

I have figured out how to tie up an event handler on a SELECT element using an ugly cast of the event to any.

Is it possible to retrieve the value in a type-safe manner without casting to any?

import React = require('react');

interface ITestState {
    selectedValue: string;
}

export class Test extends React.Component<{}, ITestState> {

    constructor() {
        super();
        this.state = { selectedValue: "A" };
    }

    change(event: React.FormEvent) {
        console.log("Test.change");
        console.log(event.target); // in chrome => <select class="form-control" id="searchType" data-reactid=".0.0.0.0.3.1">...</select>

        // Use cast to any works but is not type safe
        var unsafeSearchTypeValue = ((event.target) as any).value;

        console.log(unsafeSearchTypeValue); // in chrome => B

        this.setState({
            selectedValue: unsafeSearchTypeValue
        });
    }

    render() {
        return (
            <div>
                <label htmlFor="searchType">Safe</label>
                <select className="form-control" id="searchType" onChange={ e => this.change(e) } value={ this.state.selectedValue }>
                    <option value="A">A</option>
                    <option value="B">B</option>
                </select>
                <h1>{this.state.selectedValue}</h1>
            </div>
        );
    }
}

JSX:

<select value={ this.state.foo } onChange={this.handleFooChange}>
    <option value="A">A</option>
    <option value="B">B</option>
</select>

TypeScript:

private handleFooChange = (event: React.FormEvent<HTMLSelectElement>) => {
    const element = event.target as HTMLSelectElement;
    this.setState({ foo: element.value });
}

I tried using React.FormEvent<HTMLSelectElement> but it led to an error in the editor, even though there is no EventTarget visible in the code:

The property 'value' does not exist on value of type 'EventTarget'

Then I changed React.FormEvent to React.ChangeEvent and it helped:

private changeName(event: React.ChangeEvent<HTMLSelectElement>) {
    event.preventDefault();
    this.props.actions.changeName(event.target.value);
}

Update: the official type-definitions for React have been including event types as generic types for some time now, so you now have full compile-time checking, and this answer is obsolete.

Is it possible to retrieve the value in a type-safe manner without casting to any?

Yes. If you are certain about the element your handler is attached to, you can do:

<select onChange={ e => this.selectChangeHandler(e) }>
    ...
</select>

private selectChangeHandler(e: React.FormEvent)
{
    var target = e.target as HTMLSelectElement;
    var intval: number = target.value; // Error: 'string' not assignable to 'number'
}

Live demo

The TypeScript compiler will allow this type-assertion, because an HTMLSelectElement is an EventTarget. After that, it should be type-safe, because you know that e.target is an HTMLSelectElement, because you just attached your event handler to it.

However, to guarantee type-safety (which, in this case, is relevant when refactoring), it is also needed to check the actual runtime-type:

if (!(target instanceof HTMLSelectElement))
{
    throw new TypeError("Expected a HTMLSelectElement.");
}

As far as I can tell, this is currently not possible - a cast is always needed.

To make it possible, the .d.ts of react would need to be modified so that the signature of the onChange of a SELECT element used a new SelectFormEvent. The new event type would expose target, which exposes value. Then the code could be typesafe.

Otherwise there will always be the need for a cast to any.

I could encapsulate all that in a MYSELECT tag.

In addition to @thoughtrepo's answer:

Until we do not have definitely typed events in React it might be useful to have a special target interface for input controls:

export interface FormControlEventTarget extends EventTarget{
    value: string;
}

And then in your code cast to this type where is appropriate to have IntelliSense support:

 import {FormControlEventTarget} from "your.helper.library"

 (event.target as FormControlEventTarget).value;

it works:

type HtmlEvent = React.ChangeEvent<HTMLSelectElement>

const onChange: React.EventHandler<HtmlEvent> = 
   (event: HtmlEvent) => { 
       console.log(event.target.value) 
   }

The easiest way is to add a type to the variable that is receiving the value, like this:

var value: string = (event.target as any).value;

Or you could cast the value property as well as event.target like this:

var value = ((event.target as any).value as string);

Edit:

Lastly, you can define what EventTarget.value is in a separate .d.ts file. However, the type will have to be compatible where it's used elsewhere, and you'll just end up using any again anyway.

globals.d.ts

interface EventTarget {
    value: any;
}

In my case onChange event was typed as React.ChangeEvent:

onChange={ (e: React.ChangeEvent<HTMLSelectElement>) => {
           console.warn('onChange TextInput value: ' + e.target.value);
           } 
         }