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What are rest parameters and default parameters?

rest parameters and default parameters

1. Rest Parameters (...)

  • Rest parameters allow a function to accept an indefinite number of arguments as an array.
  • Syntax uses three dots (...) before the parameter name.

✅ Example:

function sum(...numbers) {
  return numbers.reduce((acc, num) => acc + num, 0);
}

console.log(sum(1, 2, 3));       // 6
console.log(sum(10, 20, 30, 40)); // 100

🔹 Here, numbers becomes an array of all passed arguments.

Key Points:

  • Rest parameter must be the last parameter in the function.
  • It collects extra arguments into a single array.

2. Default Parameters

  • Default parameters allow you to give default values to function parameters, if no value or undefined is passed.

✅ Example:

function greet(name = "Guest") {
  console.log("Hello, " + name);
}

greet("Himanshu"); // Hello, Himanshu
greet();           // Hello, Guest

🔹 If no argument is passed, name defaults to "Guest".

Key Points:

  • Works only if the argument is undefined (not if it’s null).
  • Can be any value: string, number, object, function.

⚡ Combined Example (Rest + Default)

function introduce(greeting = "Hi", ...names) {
  console.log(greeting + ", " + names.join(", "));
}

introduce("Hello", "Ram", "Shyam", "Geeta");
// Hello, Ram, Shyam, Geeta

introduce(undefined, "Aman", "Ravi");
// Hi, Aman, Ravi

👉 So in short:

  • Rest parameters → collect multiple arguments into an array.
  • Default parameters → set a fallback value if argument not provided.
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