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Constructor

Constructor

Introduction

The constructor method is a special method that runs automatically when a new instance is created with new. It’s where you set up instance properties and perform initialization.

Basic Constructor

class User {
  constructor(name, email) {
    this.name = name;
    this.email = email;
    this.createdAt = new Date();
  }
}

const user = new User('Alice', 'alice@email.com');
console.log(user.name);      // 'Alice'
console.log(user.createdAt); // Date object

Default Values

class Config {
  constructor(theme = 'dark', lang = 'en') {
    this.theme = theme;
    this.lang = lang;
    this.version = '1.0.0';
  }
}

const defaultConfig = new Config();
console.log(defaultConfig.theme); // 'dark'

const customConfig = new Config('light', 'fr');
console.log(customConfig.theme); // 'light'

Validation in Constructor

class BankAccount {
  constructor(owner, initialBalance = 0) {
    if (!owner) throw new Error('Owner is required');
    if (initialBalance < 0) throw new Error('Balance cannot be negative');

    this.owner = owner;
    this.balance = initialBalance;
  }
}

// Valid
const account = new BankAccount('Alice', 1000);

// Throws
const invalid = new BankAccount('', -500); // Error!

super() in Subclass Constructors

class Animal {
  constructor(name) {
    this.name = name;
  }
}

class Dog extends Animal {
  constructor(name, breed) {
    super(name); // Must call super before using this
    this.breed = breed;
  }
}

const rex = new Dog('Rex', 'Labrador');
console.log(rex.name);  // 'Rex'
console.log(rex.breed); // 'Labrador'

super() Requirements

Must call super() before accessing this
If a subclass defines a constructor, it must call super()
If no constructor is defined in a subclass, super() is automatically called

class Animal {
  constructor(name) { this.name = name; }
}

class Dog extends Animal {
  // ❌ constructor(name, breed) { this.breed = breed; }
  // ReferenceError: Must call super before accessing this

  constructor(name, breed) {
    super(name); // ✅ Required
    this.breed = breed;
  }
}

Default Constructor

If you don’t define a constructor, JavaScript provides a default one:

class User {
  // Default constructor is equivalent to:
  // constructor() {}
  greet() { return 'Hello!'; }
}

const user = new User(); // works fine
console.log(user.greet()); // 'Hello!'

Summary

constructor() runs once when a new instance is created
Default values and validation can be done in the constructor
Subclass constructors must call super() before this
Without a constructor, JS provides a default one

Further Reading