Default parameters in Javascript | Lecture 6

 

In JavaScript, default parameters allow you to initialize function parameters with default values if no value or undefined is passed. This feature simplifies function definitions and eliminates the need for additional checks or fallback logic inside the function body.

Syntax

function functionName(param1 = defaultValue1, param2 = defaultValue2) {

  // Function body

}

Examples

1. Basic Default Parameters

function greet(name = "Guest") {

  console.log(`Hello, ${name}!`);

}

 

greet("Alice"); // Output: "Hello, Alice!"

greet();        // Output: "Hello, Guest!"

2. Using Expressions as Default Values

function calculateArea(length = 1, width = length * 2) {

  return length * width;

}

 

console.log(calculateArea(5));    // Output: 50 (5 * 10)

console.log(calculateArea());     // Output: 2 (1 * 2)

3. Default Parameters and undefined

If undefined is passed explicitly, the default value is used:

function sayHello(name = "World") {

  console.log(`Hello, ${name}!`);

}

 

sayHello(undefined); // Output: "Hello, World!"

sayHello(null);      // Output: "Hello, null!" (null overrides the default)

4. Functions as Default Values

You can use a function to compute the default value dynamically:

function getDefaultName() {

  return "Anonymous";

}

 

function welcome(user = getDefaultName()) {

  console.log(`Welcome, ${user}!`);

}

 

welcome("John"); // Output: "Welcome, John!"

welcome();       // Output: "Welcome, Anonymous!"

5. Default Parameters with Destructuring

Default parameters can work with object and array destructuring:

// Object Destructuring

function displayUser({ name = "User", age = 18 } = {}) {

  console.log(`Name: ${name}, Age: ${age}`);

}

 

displayUser({ name: "Alice", age: 25 }); // Output: "Name: Alice, Age: 25"

displayUser();                           // Output: "Name: User, Age: 18"

 

// Array Destructuring

function sum([a = 0, b = 0] = []) {

  return a + b;

}

 

console.log(sum([5, 10])); // Output: 15

console.log(sum());        // Output: 0

Notes

  • Default parameters are evaluated at call time, not at function definition time. This means the default value can depend on previous parameters:

function multiply(a, b = a * 2) {

  return a * b;

}

 

console.log(multiply(3)); // Output: 18 (3 * (3 * 2))

  • You cannot skip arguments when calling a function with default parameters; instead, explicitly pass undefined to use a default value:

function greet(first = "Hello", second = "World") {

  console.log(`${first}, ${second}!`);

}

 

greet("Hi", undefined); // Output: "Hi, World!"

Default parameters enhance function flexibility and improve code readability by reducing the need for manual checks and fallback logic.

 

Comments

Post a Comment

Popular posts from this blog

Classes and Objects in Javascript | Lecture 10

  In JavaScript, classes are templates for creating objects, and objects are instances of classes. Classes encapsulate data and methods to model real-world entities. Declaring a Class A class is defined using the class keyword, and it typically contains: Constructor : A special method for initializing the object. Methods : Functions defined within the class. Example: Creating a Class and an Object class Person {   // Constructor to initialize the object   constructor(name, age) {     this.name = name; // Instance property     this.age = age;   }     // Method   greet() {     return `Hello, my name is ${this.name} and I am ${this.age} years old.`;   } }   // Create an object (instance) of the class const person1 = new Person("John", 30);   // Access properties and methods console.log(person1.name); // Output: John console.log...
Read more

Destructuring in JavaScript | Lecture 5

  Destructuring in JavaScript is a syntax for unpacking values from arrays or properties from objects into distinct variables. This feature simplifies extracting data, making your code cleaner and more readable. Here are examples of destructuring for both arrays and objects: 1. Destructuring Arrays const fruits = ['apple', 'banana', 'cherry'];   // Basic Destructuring const [first, second] = fruits; console.log(first); // "apple" console.log(second); // "banana"   // Skipping Elements const [, , third] = fruits; console.log(third); // "cherry"   // Default Values const [a, b, c = 'default fruit'] = ['mango', 'pear']; console.log(c); // "default fruit"   // Rest Pattern const [d, ...rest] = fruits; console.log(rest); // ["banana", "cherry"] 2. Destructuring Objects const user = {   name: 'John',   age: 30,   count...
Read more

ECMA Script 6 Introduction of topics | Lecture 1

Image
ECMA Script 6 Introduction of topics 1. Let and Const let : Block-scoped variable. const : Block-scoped constant. let x = 10; x = 20; // Allowed   const y = 30; // y = 40; // Error: Assignment to constant variable   2. Arrow Functions Concise syntax for defining functions. // Regular function function add(a, b) {   return a + b; }   // Arrow function const add = (a, b) => a + b;   console.log(add(5, 3)); // Output: 8   3. Template Literals For string interpolation and multiline strings. const name = "John"; const greeting = `Hello, ${name}!`; console.log(greeting); // Output: Hello, John!   const multiline = `This is a multiline string.`; console.log(multiline);   4. Default Parameters Default values for function parameters. function greet(name = "Guest") {   return `Hello, ${name}!`; }   console.log(greet()); // Output: Hello, Guest! console.log(greet(...
Read more