TypeScript Construct Signatures

TypeScript Construct Signatures defines the shape of a constructor function for creating instances of a class or object type. This signature specifies the parameters that the constructor function expects when using the new operator to create objects, as well as the type of object it constructs. You can write a construct signature by adding the new keyword in front of a call signature.

Syntax:

type Constructor = new (param1: Type1, param2: Type2, ...) => ReturnType;

Parameters:

• Constructor: This is the name you give to the type that represents the constructor function.
• new: The new keyword indicates that this is a construct signature for creating new instances.
• ( and ): These parentheses enclose the parameter list for the constructor function.
• param1, param2, …: These are the names of the parameters that the constructor function expects when creating instances. Each parameter is followed by a colon and its respective type (e.g., Type1, Type2, …).

ReturnType:

This is the type of object that the constructor function constructs and returns when called with the new operator.

Example 1: In this example, we define a TypeScript class Person with a constructor that takes a name parameter and sets the name property of the instance. Inside the class, we have a greet method that logs a greeting message with the person’s name. We create an instance of the Person class using the new operator and provide “GeeksforGeeks” as the name parameter.

Javascript

// Define a class with a constructor
class Person {
    name: string;
    constructor(name: string) {
        this.name = name;
    }
    greet() {
        console.log(`Hello, my name is ${this.name}`);
    }
}

// Create an instance of the class
// using the constructor
const person = new Person("GeeksforGeeks");

// Call the function (method) on the instance
person.greet();

Output:

Example 2: In this example, we have created a constructor in a class named book and for creating an objcet of that book class we are calling a function createBook and passing the needed parameters and it returns a new object of Book.

Javascript

// Define a class for
// representing books
class Book {
    constructor(
        public title: string, public author: string) { }
}

// Define a construct
// signature for the Book class
type BookConstructor =
    new (title: string, author: string) => Book;

// Create a factory function based
// on the construct signature
function createBook(BookConstructor: BookConstructor,
    title: string, author: string) {
    return new BookConstructor(title, author);
}

// Use the factory function
// to create a book instance
const bookInstance = createBook(Book,
    "The Story of My Experiments", "Mahatma Gandhi");

// Log the book's properties
console.log("Title:", bookInstance.title);
console.log("Author:", bookInstance.author);

Output: