Functions are the main building blocks in programming. They allow you to encapsulate code into reusable blocks.

Posted At: Aug 03, 2024 - 164 Views

Functions in JavaScript and TypeScript

Functions in JavaScript and TypeScript

To declare a function, use the function keyword followed by the name of the function, parameters, and the function body.

JavaScript Example:

function greet() {
  console.log('Hello, World!');
}

greet(); // Output: Hello, World!

TypeScript Example:

function greet(): void {
  console.log('Hello, World!');
}

greet(); // Output: Hello, World!

Parameters

Functions can take parameters to work with. These are specified within the parentheses.

JavaScript Example:

function greetUser(name) {
  console.log(`Hello, ${name}!`);
}

greetUser('Alice'); // Output: Hello, Alice!

TypeScript Example:

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

greetUser('Alice'); // Output: Hello, Alice!

Default Parameters

Parameters can have default values which are used if no argument is passed.

JavaScript Example:

function greetUser(name = 'Guest') {
  console.log(`Hello, ${name}!`);
}

greetUser(); // Output: Hello, Guest!

TypeScript Example:

function greetUser(name: string = 'Guest'): void {
  console.log(`Hello, ${name}!`);
}

greetUser(); // Output: Hello, Guest!

Returning Values

Functions can return values using the return statement.

JavaScript Example:

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

console.log(add(2, 3)); // Output: 5

TypeScript Example:

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

console.log(add(2, 3)); // Output: 5

Local and Global Variables

Variables declared inside a function are local and can only be accessed within that function. Variables declared outside any function are global.

JavaScript Example:

let userName = 'Alice';

function greetUser() {
  let message = `Hello, ${userName}`;
  console.log(message);
}

greetUser(); // Output: Hello, Alice

TypeScript Example:

let userName: string = 'Alice';

function greetUser(): void {
  let message: string = `Hello, ${userName}`;
  console.log(message);
}

greetUser(); // Output: Hello, Alice

Example with Overriding Variables

JavaScript Example:

let userName = 'Alice';

function greetUser() {
  let userName = 'Bob'; // Local variable
  console.log(`Hello, ${userName}`); // Output: Hello, Bob
}

greetUser();
console.log(userName); // Output: Alice

TypeScript Example:

let userName: string = 'Alice';

function greetUser(): void {
  let userName: string = 'Bob'; // Local variable
  console.log(`Hello, ${userName}`); // Output: Hello, Bob
}

greetUser();
console.log(userName); // Output: Alice

Function Naming Conventions

  • Functions are actions, so their names usually contain verbs.
  • Common prefixes:
  • get...: returns a value.
  • calc...: calculates something.
  • create...: creates something.
  • check...: checks something and returns a boolean.

Example with Naming Conventions

JavaScript Example:

function calculateSum(a, b) {
  return a + b;
}

console.log(calculateSum(5, 7)); // Output: 12

TypeScript Example:

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

console.log(calculateSum(5, 7)); // Output: 12

Summary

  • Functions help avoid code repetition.
  • Functions can accept parameters and return values.
  • Local variables are accessible only within the function they are declared.
  • Global variables are accessible from any function.
  • Default parameters provide fallback values.
  • Naming functions appropriately makes code more readable and maintainable.

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