How many parameters should a function have?

Best practice: 0-3. If you need more, use an object parameter: function createUser({ name, age, email }) { ... }. This also makes the call self-documenting: createUser({ name: 'Alice', age: 30 }).

What's the difference between parameters and arguments?

Parameters are the names listed in the function definition (like placeholders). Arguments are the actual values passed when calling the function. Think: parameters = definition, arguments = invocation.

Can I modify the arguments of a function?

Yes, you can reassign parameter variables inside the function. For primitives, this doesn't affect the original value (pass by value). For objects, mutating properties affects the original, but reassigning the entire parameter does not.

Should I use the 'arguments' object?

No — use rest parameters (...args) instead. The arguments object is not a real array, doesn't work in arrow functions, and is considered legacy. Rest parameters are clearer and more powerful.

JavaScript Function Parameters

Pass data into functions. Default values, rest params, and more.

Parameters & Arguments

Parameters are placeholders. Arguments are the actual values.

// 'a' and 'b' are PARAMETERS (in definition)
function add(a, b) {
  return a + b;
}

// 5 and 3 are ARGUMENTS (in call)
add(5, 3); // 8

a =

b =

add(5, 3) → 8

// Missing arguments become undefined:
function greet(name, greeting) {
  console.log(greeting + ", " + name);
}
greet("Alice"); // "undefined, Alice"

// Extra arguments are ignored:
greet("Alice", "Hello", "extra"); // "Hello, Alice"

Default Values

Provide fallback values for missing arguments.

function greet(name = "World", greeting = "Hello") {
  return `${greeting}, ${name}!`;
}

greet();              // "Hello, World!"
greet("Alice");       // "Hello, Alice!"
greet("Alice", "Hi"); // "Hi, Alice!"

// Default only applies when undefined:
function test(x = 10) {
  return x;
}
test(undefined); // 10 — uses default
test(null);      // null — NOT undefined
test(0);         // 0 — NOT undefined
test("");        // "" — NOT undefined

Defaults ONLY trigger for undefined, not for null, 0, or empty string.

1 / 2

Rest Parameters

Collect any number of arguments into an array.

// ...rest collects remaining arguments:
function sum(...numbers) {
  let total = 0;
  for (const n of numbers) {
    total += n;
  }
  return total;
}

sum(1, 2, 3);     // 6
sum(1, 2, 3, 4, 5); // 15
sum();             // 0

// Rest must be last parameter:
function tag(name, ...attributes) {
  console.log(name);       // "div"
  console.log(attributes); // ["id", "class"]
}
tag("div", "id", "class");

// ✗ This is invalid:
// function bad(...rest, last) {} // SyntaxError

arguments (old)

Array-like, not a real array. No .map(), .filter(). Avoid in modern code.

...rest (modern)

A real Array. Has all array methods. Clear and explicit.

Pass by Value vs Reference

Primitives copy. Objects share.

// Primitives: pass by value (copy)
function double(x) {
  x = x * 2;
  console.log("inside:", x); // 20
}
let num = 10;
double(num);
console.log("outside:", num); // 10 ← unchanged!

Primitives (numbers, strings, booleans) are copied. Changes inside don't affect the original.

1 / 3

FAQ

Common questions about parameters.

// Missing arguments become undefined: function greet(name, greeting) { console.log(greeting + ", " + name); } greet("Alice"); // "undefined, Alice" // Extra arguments are ignored: greet("Alice", "Hello", "extra"); // "Hello, Alice"

function greet(name = "World", greeting = "Hello") { return `${greeting}, ${name}!`; } greet(); // "Hello, World!" greet("Alice"); // "Hello, Alice!" greet("Alice", "Hi"); // "Hi, Alice!"

// Default only applies when undefined: function test(x = 10) { return x; } test(undefined); // 10 — uses default test(null); // null — NOT undefined test(0); // 0 — NOT undefined test(""); // "" — NOT undefined

// ...rest collects remaining arguments: function sum(...numbers) { let total = 0; for (const n of numbers) { total += n; } return total; } sum(1, 2, 3); // 6 sum(1, 2, 3, 4, 5); // 15 sum(); // 0

// Rest must be last parameter: function tag(name, ...attributes) { console.log(name); // "div" console.log(attributes); // ["id", "class"] } tag("div", "id", "class"); // ✗ This is invalid: // function bad(...rest, last) {} // SyntaxError