Typescript: React event types

Question

What is the correct type for React events. Initially I just used any for the sake of simplicity. Now, I am trying to clean things up and avoid use of any completely.

So in a simple form like this:

export interface LoginProps {
  login: {
    [k: string]: string | Function
    uname: string
    passw: string
    logIn: Function
  }
}
@inject('login') @observer
export class Login extends Component<LoginProps, {}> {
  update = (e: React.SyntheticEvent<EventTarget>): void => {
    this.props.login[e.target.name] = e.target.value
  }
  submit = (e: any): void => {
    this.props.login.logIn()
    e.preventDefault()
  }
  render() {
    const { uname, passw } = this.props.login
    return (
      <div id='login' >
        <form>
          <input
            placeholder='Username'
            type="text"
            name='uname'
            value={uname}
            onChange={this.update}
          />
          <input
            placeholder='Password'
            type="password"
            name='passw'
            value={passw}
            onChange={this.update}
          />
          <button type="submit" onClick={this.submit} >
            Submit
          </button>
        </form>
      </div>
    )
  }
}

What type do I use here as event type?

React.SyntheticEvent<EventTarget> does not seem to be working as I get an error that name and value do not exist on target.

More generalised answer for all events would be really appreciated.

Thanks

Answer 1

The SyntheticEvent interface is generic:

interface SyntheticEvent<T> {
    ...
    currentTarget: EventTarget & T;
    ...
}

(Technically the currentTarget property is on the parent BaseSyntheticEvent type.)

And the currentTarget is an intersection of the generic constraint and EventTarget.
Also, since your events are caused by an input element you should use the ChangeEvent (in definition file, the react docs).

Should be:

update = (e: React.ChangeEvent<HTMLInputElement>): void => {
    this.props.login[e.currentTarget.name] = e.currentTarget.value
}

(Note: This answer originally suggested using React.FormEvent. The discussion in the comments is related to this suggestion, but React.ChangeEvent should be used as shown above.)

Answer 2

The problem is not with the Event type, but that the EventTarget interface in typescript only has 3 methods:

interface EventTarget {
    addEventListener(type: string, listener: EventListenerOrEventListenerObject, useCapture?: boolean): void;
    dispatchEvent(evt: Event): boolean;
    removeEventListener(type: string, listener: EventListenerOrEventListenerObject, useCapture?: boolean): void;
}

interface SyntheticEvent {
    bubbles: boolean;
    cancelable: boolean;
    currentTarget: EventTarget;
    defaultPrevented: boolean;
    eventPhase: number;
    isTrusted: boolean;
    nativeEvent: Event;
    preventDefault(): void;
    stopPropagation(): void;
    target: EventTarget;
    timeStamp: Date;
    type: string;
}

So it is correct that name and value don't exist on EventTarget. What you need to do is to cast the target to the specific element type with the properties you need. In this case it will be HTMLInputElement.

update = (e: React.SyntheticEvent): void => {
    let target = e.target as HTMLInputElement;
    this.props.login[target.name] = target.value;
}

Also for events instead of React.SyntheticEvent, you can also type them as following: Event, MouseEvent, KeyboardEvent...etc, depends on the use case of the handler.

The best way to see all these type definitions is to checkout the .d.ts files from both typescript & react.

Also check out the following link for more explanations: Why is Event.target not Element in Typescript?