Parameters

2 min read | JavaScript Tutorial

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Function Parameters

Parameters act as placeholders for values passed to a function when it is called.

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

console.log(add(5, 3));  // 8
console.log(add(10, 20)); // 30

Default Parameter Values

ES6 introduced default parameters, which are used when no argument or undefined is passed.

function greet(name = "Guest") {
  return "Hello, " + name + "!";
}

console.log(greet("Alice")); // Hello, Alice!
console.log(greet());        // Hello, Guest!

Multiple Defaults

You can set defaults for as many parameters as needed.

function createUser(name = "User", role = "viewer", active = true) {
  return { name, role, active };
}

console.log(createUser("Bob", "admin"));
// { name: "Bob", role: "admin", active: true }

Rest Parameters

Rest parameters (...args) collect all remaining arguments into a real array.

function sumAll(...numbers) {
  return numbers.reduce((total, n) => total + n, 0);
}

console.log(sumAll(1, 2, 3, 4, 5)); // 15
console.log(sumAll(10, 20));         // 30

Rest with Named Parameters

Rest must always be the last parameter.

function listFruits(category, ...items) {
  console.log("Category: " + category);
  console.log("Items: " + items.join(", "));
}

listFruits("tropical", "mango", "papaya", "coconut");
// Category: tropical
// Items: mango, papaya, coconut

The arguments Object

In non-arrow functions, the arguments object holds all passed arguments (array-like, not a real array).

function showArgs() {
  for (let i = 0; i < arguments.length; i++) {
    console.log("Arg " + i + ": " + arguments[i]);
  }
}

showArgs("a", "b", "c");

Concepts Explained