JavaScript Function Parameters


JavaScript function parameters allow you to pass data into functions, making them dynamic, reusable, and powerful.



1. Basic Parameters


Function parameters are defined in the function declaration. When calling the function, you pass arguments.

Example:

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

greet("Alice");

Output

Hello, Alice


2. Multiple Parameters


You can define more than one parameter.

Example:

function add(a, b) {
  console.log("Sum:", a + b);
}

add(5, 10);

Output

Sum: 15


3. Default Parameters (ES6+)


Default values can be set for parameters to avoid undefined.

Example:

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

greet();         // Uses default
greet("John");   // Uses provided value

Output

Welcome, Guest
Welcome, John


4. Rest Parameters (...)


Use rest parameters to pass any number of arguments. They are gathered into an array.

Example:

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

console.log(sumAll(1, 2, 3, 4, 5));

Output

15


5. Parameter Destructuring


You can extract object or array properties directly in the parameter list.

Example (Object Destructuring):

function printUser({ name, age }) {
  console.log(name + " is " + age + " years old.");
}

printUser({ name: "Emma", age: 30 });

Output

Emma is 30 years old.


6. Arguments Object (Legacy Way)


Inside regular functions, arguments is an array-like object containing all passed arguments.

Example:

function showArgs() {
  console.log(arguments);
}

showArgs(1, 2, 3);

Output

[Arguments] { '0': 1, '1': 2, '2': 3 }


Summary Table – JavaScript Function Parameter Types

Type Syntax Example Use Case
Basic function greet(name) One or more fixed parameters
Default function greet(name = "Guest") Fallback value
Rest (...) function sum(...nums) Accept any number of arguments
Destructuring function show({name, age}) Directly extract from object/array
Arguments object function test() { console.log(arguments) } Capture all args in regular functions