Published lesson · 30 minutes

Respond to input with state

Objective: Use React state to keep an input value and its visible result in sync after interaction.

Understand the idea

Respond to input with state works best when the learner can explain the reason behind each step, not just copy a result. State stores information that changes during a component lifetime; setting state asks React to render the interface again with the new value.

A resource page lets a visitor type a topic and immediately see a count of matching items. Treat the names and details as invented practice information; the example is for learning a method and does not describe a live service.

A dependable sequence is: Create state with `useState`, read the value from the input, update it in `onChange` and derive the count from the current value. Write down the important decision before you begin so another person can follow the reasoning.

Do not mutate state directly or duplicate a value that can be derived. Test clearing the input and a term that matches nothing. Compare the result with the objective, then change one thing at a time if the first attempt does not work.

Add a second filter phrase and confirm the count is derived from state. Explain one assumption that would need checking before anyone used a similar result outside the classroom.

Keep a short record of the choices you made and the evidence you used to check them. A useful practice result is understandable by another learner, stays within the stated scope and makes its limitations visible before anyone relies on it.

Worked examples

Respond to input with state: practice example

Explore how respond to input with state supports the lesson objective with a small synthetic example.

App.tsx

import { useState } from "react";
export default function SearchCount() { const [query, setQuery] = useState(""); return <label>Search resources<input value={query} onChange={(event) => setQuery(event.target.value)} /><p>Searching for: {query || "all topics"}</p></label>; }

Practice tasks

  1. Explore respond to input with state

    Use the worked example for “Respond to input with state”. Change one small detail to suit a different synthetic community situation, then explain what you expected and how you checked whether the change still met the lesson objective.

  2. Build a React practice example

    Create a small practice result that demonstrates this objective: Use React state to keep an input value and its visible result in sync after interaction. Use only synthetic information, keep the first version focused, and write down one limitation that a later version should address.

  3. Review and explain the result

    Review your “Respond to input with state” result as if another learner will use it without your explanation. Identify one strength, one exact point that may be unclear, and a revision that would make the result easier to understand or use.

Respond to input with state becomes easier when you create state with `usestate`, check the result and explain the choice to another learner.

Course overview