Prevent rerender on function prop update

Question

I have a form with several layers of child components. The state of the form is maintained at the highest level and I pass down functions as props to update the top level. The only problem with this is when the form gets very large (you can dynamically add questions) every single component reloads when one of them updates. Here's a simplified version of my code (or the codesandbox: https://codesandbox.io/s/636xwz3rr):

const App = () => {
  return <Form />;
}

const initialForm = {
  id: 1,
  sections: [
    {
      ordinal: 1,
      name: "Section Number One",
      questions: [
        { ordinal: 1, text: "Who?", response: "" },
        { ordinal: 2, text: "What?", response: "" },
        { ordinal: 3, text: "Where?", response: "" }
      ]
    },
    {
      ordinal: 2,
      name: "Numero dos",
      questions: [
        { ordinal: 1, text: "Who?", response: "" },
        { ordinal: 2, text: "What?", response: "" },
        { ordinal: 3, text: "Where?", response: "" }
      ]
    }
  ]
};

const Form = () => {
  const [form, setForm] = useState(initialForm);

  const updateSection = (idx, value) => {
    const { sections } = form;
    sections[idx] = value;
    setForm({ ...form, sections });
  };

  return (
    <>
      {form.sections.map((section, idx) => (
        <Section
          key={section.ordinal}
          section={section}
          updateSection={value => updateSection(idx, value)}
        />
      ))}
    </>
  );
};

const Section = props => {
  const { section, updateSection } = props;

  const updateQuestion = (idx, value) => {
    const { questions } = section;
    questions[idx] = value;
    updateSection({ ...section, questions });
  };

  console.log(`Rendered section "${section.name}"`);

  return (
    <>
      <div style={{ fontSize: 18, fontWeight: "bold", margin: "24px 0" }}>
        Section name:
        <input
          type="text"
          value={section.name}
          onChange={e => updateSection({ ...section, name: e.target.value })}
        />
      </div>
      <div style={{ marginLeft: 36 }}>
        {section.questions.map((question, idx) => (
          <Question
            key={question.ordinal}
            question={question}
            updateQuestion={v => updateQuestion(idx, v)}
          />
        ))}
      </div>
    </>
  );
};

const Question = props => {
  const { question, updateQuestion } = props;

  console.log(`Rendered question #${question.ordinal}`);

  return (
    <>
      <div>{question.text}</div>
      <input
        type="text"
        value={question.response}
        onChange={e =>
          updateQuestion({ ...question, response: e.target.value })
        }
      />
    </>
  );
};

I've tried using useMemo and useCallback, but I can't figure out how to make it work. The problem is passing down the function to update its parent. I can't figure out how to do that without updating it every time the form updates.

I can't find a solution online anywhere. Maybe I'm searching for the wrong thing. Thank you for any help you can offer!

Solution

Using Andrii-Golubenko's answer and this article React Optimizations with React.memo, useCallback, and useReducer I was able to come up with this solution: https://codesandbox.io/s/myrjqrjm18

Notice how the console log only shows re-rendering of components that have changed.

Answer 1

1. Use React feature React.memo for functional components to prevent re-render if props not changed, similarly to PureComponent for class components.
2. When you pass callback like that:

<Section
    ...
    updateSection={value => updateSection(idx, value)}
/>

your component Section will rerender each time when parent component rerender, even if other props are not changed and you use React.memo. Because your callback will re-create each time when parent component renders. You should wrap your callback in useCallback hook.

3. Using useState is not a good decision if you need to store complex object like initialForm. It is better to use useReducer;

Here you could see working solution: https://codesandbox.io/s/o10p05m2vz