Registering Providers

Part of the TypeScript and NestJS section of Coddy's Node.js journey. Lesson 7 of 17.

Nest only creates the classes a module lists in providers. A provider can ask for another provider, and every one of them has to be registered.

@Injectable()
export class OrdersService {
    constructor(private readonly mailer: MailerService) {}
}

If MailerService is missing from the module's providers, the app does not start, and Nest says exactly what it could not find:

Nest can't resolve dependencies of the OrdersService (?).
Please make sure that the argument MailerService at index [0]
is available in the OrdersModule module.

The ? marks the missing constructor argument, and the message names the class and the module to fix.

Challenge

Easy

The starter adds ClockService. Ask for it in TasksService and give every new task createdAt: this.clock.now().
Register ClockService in TasksModule.
Send POST /tasks with a title. Then take ClockService out of providers, press Run, read the error, and put it back.

Try it yourself

import { Injectable, NotFoundException } from '@nestjs/common';

export interface Task {
    id: number;
    title: string;
    done: boolean;
    createdAt?: string;
}

@Injectable()
export class TasksService {
    private tasks: Task[] = [
        { id: 1, title: 'Write the README', done: true, createdAt: '2026-01-05T09:00:00.000Z' },
        { id: 2, title: 'Fix the login bug', done: false, createdAt: '2026-01-06T14:30:00.000Z' },
        { id: 3, title: 'Plan the sprint', done: false, createdAt: '2026-01-07T08:15:00.000Z' },
    ];

    findAll(done?: string): Task[] {
        if (done === undefined) {
            return this.tasks;
        }
        return this.tasks.filter((task) => task.done === (done === 'true'));
    }

    findOne(id: number): Task {
        const task = this.tasks.find((task) => task.id === id);
        if (!task) {
            throw new NotFoundException(`Task ${id} not found`);
        }
        return task;
    }

    create(title: string): Task {
        const id = Math.max(0, ...this.tasks.map((task) => task.id)) + 1;
        const task = { id, title, done: false };
        this.tasks.push(task);
        return task;
    }

    remove(id: number): void {
        this.findOne(id);
        this.tasks = this.tasks.filter((task) => task.id !== id);
    }
}