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Constructors

Q: What is the role of the 'new' keyword?

Answer: The 'new' keyword creates a new empty object, sets the 'this' context of the constructor to that new object, links it to the constructor's prototype, and returns the new object..

Q: Why should constructor functions start with a capital letter?

Answer: This is a naming convention to signal to other developers that the function is intended to be called with the 'new' keyword..

Q: What happens if you call a constructor without the 'new' keyword?

Answer: In non-strict mode, 'this' will refer to the global object (window), likely adding properties to it accidentally. In strict mode, 'this' will be undefined and throw an error..

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Expert Answer & Key Takeaways

Mastering Constructors is essential for high-fidelity technical architecture and senior engineering roles in 2026.

JavaScript Object Constructors

Sometimes we need a blueprint for creating many objects of the same "type". The way to create an "object type", is to use an object constructor function.

1. Creating a Constructor Function

It is considered good practice to name constructor functions with an uppercase first letter.

function Person(first, last, age, eye) {
  this.firstName = first;
  this.lastName = last;
  this.age = age;
  this.eyeColor = eye;
}

2. The `new` Keyword

In the constructor function, this does not have a value. It is a substitute for the new object. The value of this will become the new object when a new object is created.

const myFather = new Person("John", "Doe", 50, "blue");
const myMother = new Person("Sally", "Rally", 48, "green");

3. Adding a Property to a Constructor

You cannot add a new property to an existing object constructor the same way you add a new property to an existing object:

Person.nationality = "English"; // This will NOT work

To add a new property to a constructor, you must add it inside the constructor function, or use prototypes.

4. Adding a Method to a Constructor

You can also add methods to your constructor function:

function Person(first, last) {
  this.firstName = first;
  this.lastName = last;
  this.name = function() {
    return this.firstName + " " + this.lastName;
  };
}

[!IMPORTANT] Every time you create a new object instance using this constructor, a fresh copy of the name function is created in memory. To share the same function across all instances, you should use the prototype.

Constructors

Expert Answer & Key Takeaways

JavaScript Object Constructors

1. Creating a Constructor Function

2. The `new` Keyword

3. Adding a Property to a Constructor

4. Adding a Method to a Constructor

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Constructors

Expert Answer & Key Takeaways

JavaScript Object Constructors

1. Creating a Constructor Function

2. The new Keyword

3. Adding a Property to a Constructor

4. Adding a Method to a Constructor

Top Interview Questions

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2. The `new` Keyword