Why Declaring functions and classes in Typescript? - Purpose & Use Cases

The Big Idea

What if you could write instructions once and use them everywhere without mistakes?

The Scenario

Imagine you need to repeat the same set of instructions many times in your code, like calculating a total price or creating a user profile. Without functions or classes, you'd have to write the same lines over and over again everywhere.

The Problem

Writing the same code repeatedly is slow and tiring. It's easy to make mistakes or forget to update all copies when something changes. Managing complex data without classes can become confusing and messy.

The Solution

Functions let you write a set of instructions once and reuse them anytime, making your code cleaner and easier to fix. Classes help you organize related data and actions together, like a blueprint for creating many similar objects.

Before vs After

✗ Before

let price1 = 10; let price2 = 20; let total = price1 + price2; // repeated everywhere

✓ After

function sum(a: number, b: number): number { return a + b; } let total = sum(10, 20);

What It Enables

It lets you build clear, reusable, and organized code that's easy to maintain and expand.

Real Life Example

Think of a class as a recipe for making many cakes. Instead of writing the recipe each time, you just follow the class blueprint to create each cake (object) with its own flavor and size.

Key Takeaways

Functions save time by reusing code.

Classes organize data and behavior together.

Both make your code easier to read and maintain.

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)