Constructor Inputs

Part of the TypeScript and NestJS section of Coddy's Node.js journey. Lesson 8 of 24.

A constructor runs when an instance is created. Its arguments provide values that the object will need later. TypeScript can store an argument on the instance automatically.

class Price {
    constructor(private readonly tax: number) {}
    total(amount: number) { return amount * (1 + this.tax); }
}
const price = new Price(0.2);
price.total(10); // 12

private makes the field accessible only inside the class. readonly prevents assigning a different value after construction; it does not freeze a referenced object. Together, these words before a constructor parameter create and initialize this.tax. No second assignment is needed. Nest uses this shorthand to keep objects it supplies to a controller.

Challenge

Easy

In src/exercise.ts, add a constructor to Label. It receives a string prefix and stores it using TypeScript’s private readonly parameter shorthand. Keep render unchanged.

Try it: GET /result returns ["Note: Read","Task: Write"]. Each instance keeps the prefix passed at construction.

Try it yourself

export class Label {
    // Add a constructor that stores the supplied prefix.
    render(text: string) {
        return this.prefix + text;
    }
}

All lessons in TypeScript and NestJS

1Nest Basics

Classes and Methods Hello, Nest Registering a Controller

Route Parameters

Query Parameters

Creating Tasks

Status Codes and Errors

4Guards, Filters, Interceptors

Guards

Reading an HTTP Exception Exception Filters A Value in an Observable

Observing with tap

Interceptors

The Same API

2Providers

Constructor InputsA Service Registering Providers

A Second Module

Configuration Values

3Validation

DTOs

A Rule on a DTO Field

The Validation Pipe

Unknown Fields

Parse Pipes

Practice on your own: Express playground