Constructor and fields

Learn class fields with the constructor method.

What is a constructor?

The constructor is a special method called automatically when an instance is created with new. It takes parameters and initializes the class fields with this. This way you can provide values all at once when creating an instance.

class Person {
  name: string;
  age: number;
  constructor(name: string, age: number) {
    this.name = name;
    this.age = age;
  }
}
const p = new Person("Ada", 30);

The this keyword

this refers to the instance the method belongs to. You access that instance's name field with this.name. When the parameter name and the field name are the same, you cannot distinguish them without this.

class Person {
  name: string;
  constructor(name: string) {
    this.name = name;
  }
}

Exercise Book class and constructor

Define a Book class with title: string and author: string fields. Add a constructor that takes both parameters and assigns them with this. Create a new Book instance ("TypeScript", "Ada") and print the title field.

Show example solution

class Book {
  title: string;
  author: string;
  constructor(title: string, author: string) {
    this.title = title;
    this.author = author;
  }
}

const book = new Book("TypeScript", "Ada");
console.log(book.title);

Test yourself

What is the purpose of a constructor?
- Add methods
- Define static members
- Initialize fields when a new instance is created
- Delete the class
Show answer

Initialize fields when a new instance is created — The constructor is called automatically when an instance is created with new and initializes the class fields. Deleting the class, adding methods, or defining static members are not the constructor's job.

Assign with this

constructor(name: string) {
    ____.name = name;
  }

Show answer

constructor(name: string) {
    this.name = name;
  }

Constructor parts

constructor

Constructor method
this.name = name

Field initialization
new Person("Ada", 30)

Parameterized instance creation
name: string

Parameter type

Next lesson: Methods

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