Declaring functions and classes in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a class method call

What is the output of this TypeScript code?

Typescript

class Greeter {
  greeting: string;
  constructor(message: string) {
    this.greeting = message;
  }
  greet() {
    return `Hello, ${this.greeting}`;
  }
}

const greeter = new Greeter("world");
console.log(greeter.greet());

A Error: Property 'greeting' does not exist

B Hello, undefined

C Hello, world

D Hello, message

💡 Hint

Look at how the constructor sets the greeting property and how greet() uses it.

The constructor sets the greeting property to 'world'. The greet() method returns 'Hello, ' plus the greeting property, so the output is 'Hello, world'.

❓ Predict Output

intermediate

Function return type and output

What will this TypeScript function return when called with argument 5?

Typescript

function multiplyByTwo(x: number): number {
  return x * 2;
}

console.log(multiplyByTwo(5));

A 10

B "10"

C undefined

D Error: Function must return a string

💡 Hint

Check the function's return statement and the argument passed.

The function multiplies the input number by 2 and returns it. For input 5, it returns 10.

🔧 Debug

advanced

Identify the error in this class declaration

What error does this TypeScript code produce?

Typescript

class Person {
  name: string;
  constructor(name: string) {
    this.name = name;
  }
  greet(): void {
    console.log('Hello, ' + this.names);
  }
}

const p = new Person('Alice');
p.greet();

A TypeError: Cannot read property 'names' of undefined

B SyntaxError: Unexpected token

C undefined

D ReferenceError: names is not defined

💡 Hint

Check the property name used inside the greet method.

The class has a property 'name', but the greet method tries to access 'this.names', which is undefined. This causes a TypeError at runtime.

📝 Syntax

advanced

Which option declares a valid function with default parameter?

Which of the following TypeScript function declarations is syntactically correct and sets a default value for parameter 'count'?

A function repeat(text: string, count?: number = 3): string { return text.repeat(count); }

B function repeat(text: string, count = 3): string { return text.repeat(count); }

C function repeat(text: string, count: number = 3?) { return text.repeat(count); }

D function repeat(text: string, count = 3?) { return text.repeat(count); }

💡 Hint

Default parameters are assigned with '=' without '?' after the parameter name.

Option B correctly assigns a default value to 'count'. Option B mixes optional and default syntax incorrectly. Options C and D have syntax errors with misplaced '?'.

🚀 Application

expert

Class inheritance and method overriding output

What is the output of this TypeScript code?

Typescript

class Animal {
  speak(): string {
    return 'Generic sound';
  }
}

class Dog extends Animal {
  speak(): string {
    return 'Woof';
  }
}

const pet: Animal = new Dog();
console.log(pet.speak());

A Error: Method speak not found

B Generic sound

C undefined

D Woof

💡 Hint

Think about which speak method runs when a Dog instance is assigned to an Animal type variable.

Even though the variable type is Animal, the actual object is Dog. The speak method in Dog overrides Animal's method, so 'Woof' is printed.