Declaring functions and classes in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a function named greet that returns a string.

Typescript

function greet() : [1] {
  return "Hello!";
}

Drag options to blanks, or click blank then click option'

Astring

Bnumber

Cboolean

Dvoid

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'void' as return type when the function actually returns a string.

Using 'number' or 'boolean' which do not match the returned value.

The function greet returns a string, so the return type should be string.

2fill in blank

medium

Complete the code to declare a class named Person with a constructor that takes a name parameter.

Typescript

class Person {
  name: string;
  constructor([1]: string) {
    this.name = name;
  }
}

Drag options to blanks, or click blank then click option'

AfullName

Bage

CpersonName

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different parameter name that does not match the assignment.

Forgetting to declare the parameter in the constructor.

The constructor parameter should be named name to match the property assignment this.name = name;.

3fill in blank

hard

Fix the error in the method declaration inside the class Calculator.

Typescript

class Calculator {
  add(a: number, b: number) : [1] {
    return a + b;
  }
}

Drag options to blanks, or click blank then click option'

Avoid

Bstring

Cnumber

Dboolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'void' which means no return value.

Using 'string' or 'boolean' which do not match the returned value.

The add method returns the sum of two numbers, so its return type should be number.

4fill in blank

hard

Fill both blanks to declare a class Animal with a method makeSound that returns a string.

Typescript

class [1] {
  [2]() : string {
    return "Some sound";
  }
}

Drag options to blanks, or click blank then click option'

AAnimal

BmakeSound

Csound

Dnoise

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect class or method names.

Forgetting to specify the return type.

The class name is Animal and the method name is makeSound which returns a string.

5fill in blank

hard

Fill all three blanks to declare a class Rectangle with a constructor and a method area that returns the area as a number.

Typescript

class [1] {
  width: number;
  height: number;
  constructor([2]: number, [3]: number) {
    this.width = width;
    this.height = height;
  }
  area() : number {
    return this.width * this.height;
  }
}

Drag options to blanks, or click blank then click option'

ARectangle

Bwidth

Cheight

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong parameter names that do not match the properties.

Using a class name that does not fit the context.

The class is named Rectangle. The constructor parameters are width and height to match the properties.

Practice

1. What is the main purpose of a function in TypeScript?

easy

A. To create a new type of variable

B. To store data without any behavior

C. To perform a specific task and optionally return a result

D. To define a constant value

Solution

1. Step 1: Understand what a function does

   A function is a block of code designed to perform a specific task.

2. Step 2: Recognize the role of return values

   Functions can return results after performing their task, making them reusable.

3. Final Answer:

   To perform a specific task and optionally return a result -> Option C

4. Quick Check:

   Function purpose = perform task + return result [OK]

Hint: Functions do tasks and can give back results [OK]

Common Mistakes:

• Confusing functions with variables
• Thinking functions only store data
• Believing functions cannot return values

2. Which of the following is the correct way to declare a function named greet that takes a string name and returns a string in TypeScript?

easy

A. function greet(name: string): string { return `Hello, ${name}`; }

B. function greet(name) { return 'Hello, ' + name; }

C. function greet(name: string) { console.log('Hello, ' + name); }

D. function greet(): string { return name; }

Solution

1. Step 1: Check function parameter and return types

   function greet(name: string): string { return `Hello, ${name}`; } correctly declares the parameter name as a string and the return type as string.

2. Step 2: Verify function body returns a string

   function greet(name: string): string { return `Hello, ${name}`; } returns a template string greeting the name, matching the return type.

3. Final Answer:

   function greet(name: string): string { return `Hello, ${name}`; } -> Option A

4. Quick Check:

   Correct parameter and return types = function greet(name: string): string { return `Hello, ${name}`; } [OK]

Hint: Specify parameter and return types explicitly [OK]

Common Mistakes:

• Omitting parameter types in TypeScript
• Not returning a value when return type is declared
• Using console.log instead of return

3. What will be the output of the following TypeScript code?

class Person {
  name: string;
  constructor(name: string) {
    this.name = name;
  }
  greet() {
    return `Hi, I am ${this.name}`;
  }
}
const p = new Person('Anna');
console.log(p.greet());

medium

A. Hi, I am Anna

B. Hi, I am undefined

C. Error: Property 'name' does not exist

D. Hi, I am

Solution

1. Step 1: Understand class construction

   The Person class sets this.name to the passed argument 'Anna' in the constructor.

2. Step 2: Analyze the greet method output

   The greet method returns a string including this.name, which is 'Anna'.

3. Final Answer:

   Hi, I am Anna -> Option A

4. Quick Check:

   Class property set correctly = 'Hi, I am Anna' [OK]

Hint: Constructor sets properties; methods use them [OK]

Common Mistakes:

• Forgetting to assign constructor parameter to this.name
• Expecting undefined because of missing this
• Confusing method return with console.log output

4. Identify the error in this TypeScript class declaration:

class Car {
  model: string;
  constructor(model) {
    this.model = model;
  }
  display() {
    console.log('Model: ' + this.model);
  }
}

medium

A. Property 'model' should be private

B. Missing return type for display method

C. Constructor should not have parameters

D. Missing type annotation for constructor parameter 'model'

Solution

1. Step 1: Check constructor parameter types

   In TypeScript, constructor parameters should have type annotations; here, model lacks a type.

2. Step 2: Verify other parts

   The display method does not require a return type annotation since it returns void implicitly.

3. Final Answer:

   Missing type annotation for constructor parameter 'model' -> Option D

4. Quick Check:

   Constructor parameters need types = Missing type annotation for constructor parameter 'model' [OK]

Hint: Always type constructor parameters in TypeScript [OK]

Common Mistakes:

• Omitting parameter types in constructors
• Thinking methods must always declare return types
• Assuming properties must be private by default

5. You want to create a TypeScript class Calculator with a method add that takes two numbers and returns their sum. Which code correctly implements this with proper types and usage?

hard

A. class Calculator {
add(a, b) {
return a + b;
}
}
const calc = Calculator();
console.log(calc.add(5, 3));

B. class Calculator {
add(a: number, b: number): number {
return a + b;
}
}
const calc = new Calculator();
console.log(calc.add(5, 3));

C. class Calculator {
add(a: number, b: number) {
console.log(a + b);
}
}
const calc = new Calculator();
calc.add(5, 3);

D. class Calculator {
add(a: number, b: number): number {
return a + b;
}
}
const calc = new Calculator();
console.log(calc.add('5', '3'));

Solution

1. Step 1: Check method declaration and types

   class Calculator { add(a: number, b: number): number { return a + b; } } const calc = new Calculator(); console.log(calc.add(5, 3)); declares add with parameters typed as numbers and return type number, which is correct.

2. Step 2: Verify object creation and method call

   class Calculator { add(a: number, b: number): number { return a + b; } } const calc = new Calculator(); console.log(calc.add(5, 3)); creates an instance with new Calculator() and calls add(5, 3), which returns 8.

3. Final Answer:

   Correct class with typed method and proper usage -> Option B

4. Quick Check:

   Typed method + new instance + correct call = class Calculator { add(a: number, b: number): number { return a + b; } } const calc = new Calculator(); console.log(calc.add(5, 3)); [OK]

Hint: Use 'new' to create class instances and type parameters [OK]

Common Mistakes:

• Calling class without 'new'
• Omitting parameter types
• Passing wrong argument types (like strings instead of numbers)