React · Theory

Constructor in Function Components

Theory

77/94

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’.

Diagram

Parent
    │ props (read-only)
    ▼
  Child → UI

Exam tip

useState initial ≈ class constructor state.

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’.

Short notes

DefNo constructor.
RuleInit : useState(value) or lazy fn.
RememberMount : useEffect.

React · Theory

Constructor in Function Components

← All stacks

Theory

77/94

Constructor in Function Components

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’.

Diagram

Parent
    │ props (read-only)
    ▼
  Child → UI

Exam tip

useState initial ≈ class constructor state.

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’.

Short notes

DefNo constructor.
RuleInit : useState(value) or lazy fn.
RememberMount : useEffect.

Constructor in Function Components

Example

Short notes

Questions

Constructor in Function Components

Example

Short notes

Questions