Typing React Events and Hooks

React event handlers receive typed synthetic events, and hooks expose typed state, refs, and effects. For an input change, event.currentTarget.value is a string; parse it if the application needs a number. Effects should clean up subscriptions and timers that they create.

An input's value is text, even if type="number". The parser accepts a nonnegative integer and rejects invalid input before a typed handler stores the result in state.

Casting an event value as number changes types only; it does not convert the string from an input element.

This source was compiled using TypeScript 5.8.3 with strict checking, then executed with Node.js 22.16.0. Its stdout below is the actual captured output. The core logic is runnable without a framework or a remote API; integration-specific examples appear in a separate lab when needed.

TypeScript source

function parseQuantity(text: string): number | null {
  const value = Number(text);
  return Number.isInteger(value) && value >= 0 ? value : null;
}
console.log(parseQuantity("12"));
console.log(parseQuantity("twelve"));

Verified output

12
null

Compile: npx tsc --strict --target es2022 --module commonjs typing-react-events-hooks.ts; run: node typing-react-events-hooks.js. For examples importing node: modules, install @types/node as a development dependency; for browser or React projects use the separate lab setup.

An input's value is text, even if type="number". The parser accepts a nonnegative integer and rejects invalid input before a typed handler stores the result in state.

Before editing the code, predict what the output will be for one changed input. Run it and compare your prediction with the actual result.

Integration companion: The primary example above is self-contained. The files below demonstrate a real framework, runtime, or project setup and may need additional packages or a browser.

src/Tasks.tsx

import { useState, type FormEvent } from "react";
type Todo = Readonly<{ id: number; title: string; done: boolean }>;
export default function Tasks() {
  const [tasks, setTasks] = useState<Todo[]>([]);
  const [title, setTitle] = useState("");
  function add(event: FormEvent<HTMLFormElement>) {
    event.preventDefault();
    if (!title.trim()) return;
    setTasks(current => [...current, { id: Date.now(), title: title.trim(), done: false }]);
    setTitle("");
  }
  return <section><h1>Tasks</h1><form onSubmit={add}>
    <input aria-label="Task title" value={title} onChange={e => setTitle(e.currentTarget.value)} />
    <button>Add</button></form>
    <ul>{tasks.map(task => <li key={task.id}><label>
      <input type="checkbox" checked={task.done} onChange={() => setTasks(current =>
        current.map(item => item.id === task.id ? { ...item, done: !item.done } : item))} />
      {task.title}</label></li>)}</ul>
  </section>;
}

Setup and execution

npm create vite@latest typed-tasks -- --template react-ts
cd typed-tasks
Edit src/App.tsx with the component shown above
npm install
npm run dev
npm run build

This is a real React TSX component, not a Node-only simulation. Use the React+TypeScript Vite template, and place the component in the application's src/App.tsx. Its imports require React packages and @types/react; browser behavior was not executed in this environment.

Validation: This companion requires the named external packages or browser environment. Its runtime behavior was not executed during the course build.

Casting an event value as number changes types only; it does not convert the string from an input element.

Determine whether the issue belongs to the type system, a trust boundary, or actual runtime behavior before choosing a correction.

In a real TSX example, write onChange={(event) => setText(event.currentTarget.value)} with useState<string>.

Record expected success and failure outcomes, run the code, and compare the observed behavior. Keep the exercise compatible with strict type checking.

Review the official documentation for the provider, framework or runtime, especially when installing a newer major version.

An input's value is text, even if type="number". The parser accepts a nonnegative integer and rejects invalid input before a typed handler stores the result in state.

Apply the same reasoning to the practice task, then integrate it into a real app only when the runtime and dependency setup are ready.