Functions in JavaScript – Learn Once, Use Everywhere

1.Function Definition

A function is a set of instructions that performs a particular job when we call it.

(Or)

A function in JavaScript is a block of reusable code that is designed to perform a specific task. It runs only when it is called (invoked), and it helps to reduce code repetition, improve readability, and make programs easier to manage.

Example

function makeParotta() {
    console.log("Kothu Parotta is Ready");
}

makeParotta();  // calling the function

2.Rules for Declaring a Function

1. Starts with lower case

2. No Space Allowed

3. No Special Characters are allowed except Underscore(_) and Dollar($)
   

function myFunction() { }
function _test() { }
function $demo() { }

JavaScript keywords cannot be used as function names

Reserved words like if, else, for, while, etc. cannot be used.

function if() { }   // ❌
function for() { }  // ❌

Parameters in Function

Definition

Parameters are the variables listed in the function definition. They act as placeholders to receive values when the function is called.

function add(a, b) {   // a and b are parameters
  return a + b;
}

Here, a and b are parameters.

Arguments in Function

Definition

Arguments are the actual values passed to the function when calling it.

add(5, 3);   // 5 and 3 are arguments

Here, 5 and 3 are arguments.

Simple One-Line Definition:

Parameter: Variable in function definition.

Argument: Value passed to the function.

Easy to Remember Trick:

P → Parameter → Placeholders
A → Argument → Actual values

Difference between Parameters and Arguments

Return Statement in a Function

A return statement gives the output of a function back to the caller.

(Or)

The return statement is used to send a value back from a function to the place where the function was called and stop the execution of the function.

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

let result = add(5, 3);
console.log(result);   // Output: 8

Explanation

• return a + b; → sends the value 8 back

• result receives 8

Important Points

1. A function can return only one value at a time.

2. Any code after the return statement will not execute.

3. If no return statement is used, the function returns undefined.