Intermediate Code Reading #OOP #classes #TypeScript

🏗️ Classes & Objects

3 exercises — read class definitions and choose the most professional, accurate English description. Covers constructor patterns, inheritance, and generics.

0 / 3 completed

OOP description vocabulary

"This class is responsible for managing / storing / handling…"
"[ChildClass] extends / inherits from [ParentClass]."
"It overrides the [method] from the parent class."
"The constructor initialises the [property] to [value]."
"This is a generic class — it works with any type T."

1 / 3

Read this class definition. Which description is the most accurate?

class EventEmitter {
  constructor() {
    this.listeners = {};
  }
  on(event, callback) {
    if (!this.listeners[event]) this.listeners[event] = [];
    this.listeners[event].push(callback);
  }
  emit(event, ...args) {
    (this.listeners[event] || []).forEach(cb => cb(...args));
  }
  off(event, callback) {
    this.listeners[event] = (this.listeners[event] || []).filter(cb => cb !== callback);
  }
}

2 / 3

Which sentence best describes the relationship between these two classes?

class Animal {
  constructor(name) { this.name = name; }
  speak() { return `${this.name} makes a sound.`; }
}

class Dog extends Animal {
  speak() { return `${this.name} barks.`; }
}

3 / 3

A team lead asks you to explain what this TypeScript class does. Choose the best response:

class Cache<T> {
  private store = new Map<string, { value: T; expiresAt: number }>();

  set(key: string, value: T, ttlMs: number) {
    this.store.set(key, { value, expiresAt: Date.now() + ttlMs });
  }

  get(key: string): T | undefined {
    const entry = this.store.get(key);
    if (!entry || Date.now() > entry.expiresAt) {
      this.store.delete(key);
      return undefined;
    }
    return entry.value;
  }
}