Typing React Props and State

React props represent inputs; state represents values that can change over time through React's state APIs. Use explicit prop types and infer state when the initial value is unambiguous. Never mutate a React state array and expect React to notice; use the setter to replace it with an updated value.

The old array remains length one; the new array has two elements. A React state setter can receive the new array, enabling a predictable UI update.

Pushing into an existing state array mutates it and can make updates hard to track; use immutable replacement and a stable key.

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

type Todo = Readonly<{ id: number; title: string }>;
function appendTodo(items: readonly Todo[], todo: Todo): Todo[] {
  return [...items, todo];
}
const before: Todo[] = [{ id: 1, title: "Read" }];
const after = appendTodo(before, { id: 2, title: "Write" });
console.log(before.length, after.length);

Verified output

1 2

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

The old array remains length one; the new array has two elements. A React state setter can receive the new array, enabling a predictable UI update.

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.

Pushing into an existing state array mutates it and can make updates hard to track; use immutable replacement and a stable key.

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

In a React component, use useState<Todo[]>([]) and setTodos(previous => appendTodo(previous, next)).

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.

The old array remains length one; the new array has two elements. A React state setter can receive the new array, enabling a predictable UI update.

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