Constructor in Function Components — React Technical Interview Questions & Answers

Constructor in Function Components

Constructor in Function Components

  • There is no constructor in a function component. People search this because class tutorials burned the word in. Initial state is the first argument to useState. Expensive init: useState(() => createRows()). That function runs once.
  • Mount work (subscribe, fetch, focus) is useEffect, not a constructor body. Saying ‘functional constructor’ in a viva without this correction sounds confused.
  • Constructor in Function Components — on screen: 0. First useState argument is the initial value — that is the ‘constructor’.
  Parent
    │ props (read-only)
    ▼
  Child → UI

Technical Interview Tip: useState initial ≈ class constructor state.

Code Examples for Constructor in Function Components

Example

// No constructor
import { useState } from "react";

export default function Box({ start = 0 }) {
  const [n] = useState(start);
  return <p>{n}</p>;
}

Constructor in Function Components — on screen: 0. First useState argument is the initial value — that is the ‘constructor’.

How to Explain Constructor in Function Components to an Interviewer

Explain Constructor in Function Components as if you are teaching a junior — definition, then one tiny UI.

No constructor. Init : useState(value) or lazy fn.

What does the example show on screen (or print), and what does that prove?

Constructor in Function Components — on screen: 0

What mistake do freshers make with Constructor in Function Components?

Mount : useEffect.