What are default parameters and how do you use them? Default parameters क्या हैं और कैसे use करते हैं?

Default parameters allow you to provide default values for function parameters if no argument is passed. This was introduced in ES6.

// Before ES6 - using || operator (problematic)
function greet(name) {
    name = name || 'Guest'; // 0, false, '' also trigger default
    console.log('Hello ' + name);
}

// ES6 default parameters (correct approach)
function greet2(name = 'Guest') {
    console.log('Hello ' + name);
}

greet2(); // 'Hello Guest'
greet2('John'); // 'Hello John'
greet2(null); // 'Hello null' - null is a value!
greet2(undefined); // 'Hello Guest' - undefined triggers default

// Arrow functions with defaults
const add = (a = 0, b = 0) => a + b;
add(); // 0
add(5); // 5
add(5, 3); // 8

// Default parameters can reference other parameters
function createUser(name = 'Anonymous', email = name + '@example.com') {
    return { name, email };
}
createUser('John'); // { name: 'John', email: 'John@example.com' }
createUser('Jane', 'jane@gmail.com'); // { name: 'Jane', email: 'jane@gmail.com' }

// Default parameters can be expressions
function config(timeout = 5000, retries = Math.floor(timeout / 1000)) {
    return { timeout, retries };
}
config(); // { timeout: 5000, retries: 5 }

// Default parameters can be function calls
function test(value = getDefaultValue()) {
    console.log(value);
}

// Skipping parameters (use undefined)
function multiply(a = 2, b = 3, c = 4) {
    return a * b * c;
}
multiply(undefined, 5); // 2 * 5 * 4 = 40 (a uses default)

Interview tip: Remember that null is a value and won't trigger the default, but undefined will. Also, default parameters are evaluated at call time, not at function definition time.