What is a Constructor?

A constructor() is a special method in a JavaScript class that gets executed automatically the very moment you create a new object using the new keyword.

Its primary job is to initialize the object by setting up its starting properties.

Using the Constructor

Let's say we have a Student class. We want every student to have a unique name and age when they are created. We can pass these values into the constructor.

class Student {
    constructor(studentName, studentAge) {
        // 'this' refers to the specific object being created right now
        this.name = studentName;
        this.age = studentAge;
    }

    introduce() {
        console.log(`Hi, my name is ${this.name} and I am ${this.age} years old.`);
    }
}

// The arguments here are passed directly to the constructor
const himanshu = new Student("Himanshu", 22);
const sarah = new Student("Sarah", 25);

himanshu.introduce(); // "Hi, my name is Himanshu and I am 22 years old."
sarah.introduce();    // "Hi, my name is Sarah and I am 25 years old."

The 'this' Keyword inside a Constructor

Inside a constructor, the this keyword is incredibly important. It represents the actual instance (the object) that is currently being created.

When we write this.name = studentName;, we are telling JavaScript: "Take the variable studentName that was passed in, and attach it to this specific object as a property called name."

💡 Himanshu's Tip:

You can only have one constructor method per class in JavaScript. If you try to write two constructor() functions in the same class, JavaScript will throw a SyntaxError. If you need different ways to construct an object, you handle it using default parameters or logic inside that single constructor.

Interview Questions

When is the constructor method called?
What happens if you do not define a constructor in a class?
What does the this keyword refer to inside a class constructor?