Type the onSubmit event in React (TypeScript)

Type the onSubmit event in React (TypeScript) #

Use the React.FormEvent<HTMLFormElement> type to type the onSubmit event in React. The FormEvent interface is used for onSubmit events. You can access properties on the form element the event is attached to on the currentTarget property.

App.tsx

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import React, {useState} from 'react';

const App = () => {
  const [message, setMessage] = useState('');

  // 👇️ type event as React.FormEvent<HTMLFormElement>
  const handleSubmit = (event: React.FormEvent<HTMLFormElement>) => {
    // 👇️ prevent page refresh
    event.preventDefault();

    console.log(event.currentTarget.elements);
    console.log(event.currentTarget.elements[0]);
  };

  return (
    <div>
      <form onSubmit={handleSubmit}>
        <input
          type="text"
          id="message"
          name="message"
          value={message}
          onChange={event => setMessage(event.target.value)}
        />

        <button type="submit">Submit</button>
      </form>
    </div>
  );
};

export default App;

We typed the event as React.FormEvent<HTMLFormElement> because the FormEvent type is used for onSubmit events in React.

The easiest way for you to find out what the type of an event is, is to write the event handler inline and hover over the event parameter in the function.

App.tsx

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import React, {useState} from 'react';

const App = () => {
  const [message, setMessage] = useState('');

  // 👇️ onSubmit event is written inline
  // hover over the `event` parameter with your mouse
  return (
    <div>
      <form onSubmit={event => console.log(event)}>
        <input
          type="text"
          id="message"
          name="message"
          value={message}
          onChange={event => setMessage(event.target.value)}
        />

        <button type="submit">Submit</button>
      </form>
    </div>
  );
};

export default App;

get type of onsubmit event inline

When the event is written inline, I can hover over the event parameter and it shows me what the type of the event is.

TypeScript is able to infer the type of the event when it's written inline.

This is very useful because it works with all events. Simply write a "mock" implementation of your event handler inline and hover over the event parameter to get its type.

Once you know what the type of the event is, you are able to extract your handler function and type it correctly.

Now that we know that the correct type for the onSubmit event is React.FormEvent<HTMLFormElement>, we can extract our handler function.

App.tsx

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import React, {useState} from 'react';

const App = () => {
  const [message, setMessage] = useState('');

  // 👇️ type event correctly
  const handleSubmit = (event: React.FormEvent<HTMLFormElement>) => {
    // 👇️ prevent page refresh
    event.preventDefault();

    console.log(event.currentTarget.elements);
    console.log(event.currentTarget.elements[0]);
  };

  return (
    <div>
      <form onSubmit={handleSubmit}>
        <input
          type="text"
          id="message"
          name="message"
          value={message}
          onChange={event => setMessage(event.target.value)}
        />

        <button type="submit">Submit</button>
      </form>
    </div>
  );
};

export default App;

We used the event.preventDefault() method in the handleSubmit function to prevent the page from refreshing when the form is submitted.

Note that you can use this approach to get the type of all events, not just onSubmit events.

get type of event

As long as you write the event handler function inline and hover over the event parameter, TypeScript will be able to infer the event's type.

Notice that we used the currentTarget property on the event because we want to access the element that the event listener is attached to (the form element).

The target property on the event gives us a reference to the element that triggered the event.

Type the onSubmit event in React (TypeScript)

Type the onSubmit event in React (TypeScript) #

Borislav Hadzhiev