Declaring functions and classes in Typescript

Introduction

Functions and classes help you organize your code into reusable blocks. Functions do tasks, and classes create objects with properties and actions.

When you want to repeat a task many times without rewriting code.

When you want to group related data and actions together, like a user or a car.

When you want to make your code easier to read and maintain.

When you want to create multiple similar objects with shared behavior.

When you want to separate your program into smaller, manageable parts.

Syntax

Typescript

function functionName(parameters): returnType {
  // code to run
}

class ClassName {
  propertyName: type;

  constructor(parameters) {
    // initialize properties
  }

  methodName(parameters): returnType {
    // code to run
  }
}

Functions start with the function keyword, followed by a name and parentheses.

Classes use the class keyword and can have properties, a constructor, and methods.

Examples

A simple function that says hello to a name.

Typescript

function greet(name: string): void {
  console.log(`Hello, ${name}!`);
}

A class that stores a person's name and can say hi.

Typescript

class Person {
  name: string;

  constructor(name: string) {
    this.name = name;
  }

  sayHi(): void {
    console.log(`Hi, I'm ${this.name}`);
  }
}

A function that adds two numbers and returns the result.

Typescript

function add(a: number, b: number): number {
  return a + b;
}

Sample Program

This program defines a function to multiply two numbers and a class to represent a rectangle with an area method. It prints the multiplication result and the rectangle's area.

Typescript

function multiply(x: number, y: number): number {
  return x * y;
}

class Rectangle {
  width: number;
  height: number;

  constructor(width: number, height: number) {
    this.width = width;
    this.height = height;
  }

  area(): number {
    return this.width * this.height;
  }
}

console.log(multiply(4, 5));

const rect = new Rectangle(3, 7);
console.log(rect.area());

Important Notes

Use void as the return type when a function does not return a value.

Inside classes, this refers to the current object.

Always initialize class properties in the constructor to avoid errors.

Summary

Functions perform tasks and can return results.

Classes group data and actions to create objects.

Use functions and classes to write clean, reusable code.

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)