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

2. Multiple Parameters

You can define more than one parameter.

Example:

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

add(5, 10);

Output

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

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

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

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

Summary Table – JavaScript Function Parameter Types