JavaScript Functions

Functions are Code Blocks

Functions are reusable code blocks designed to perform a particular task.

Functions are executed when they are called or invoked.

Functions are fundamental in all programming languages.

Why Use Functions?

Functions help you to:

• Reuse code (write once, run many times)
• Organize code into smaller parts
• Make code easier to read and maintain

What Does a Function Look Like?

A function can be created with the function keyword, a name, and parentheses.

The code to run is written inside curly brackets.

Example

function sayHello() { return "Hello World"; }

Functions Run When You Call Them

To run a function, you call it by using its name followed by parentheses like sayHello():

Example

function sayHello() {

  return "Hello World";

}

let message = sayHello();

JavaScript Function Syntax

Example

function
name( p1, p2, ... ) {

  // code to be executed

}

Functions are defined with the function keyword:

• followed by the function name
• followed by parentheses ( )
• followed by brackets { }

The function name follows the naming rules for variables.

Optional parameters are listed inside parentheses: ( p1, p2, ... )

Code to be executed is listed inside curly brackets: { }

Functions can return an optional value back to the caller.

Example

function multiply(a, b) {

    return a * b;

}

A Function Can Be Used Many Times

A big benefit is that you can call the same function whenever you need it.

Example

function add(a, b) {

    return a + b;

}

let sum1 = add(5, 5);

let sum2 = add(50, 50);

Local Variables

Variables declared within a JavaScript function, become LOCAL to the function.

Local variables can only be accessed from within the function.

Example

// code here can NOT use carName

function myFunction() {

  let carName = "Volvo";

  // code here CAN use carName

}

// code here can NOT use carName

Since local variables are only recognized inside their functions, variables with the same name can be used in different functions.

Functions Used as Variables

Functions can be used as variables, in all types of formulas, assignments, and calculations.

Example

let x = toCelsius(77);

let text = "The temperature is " + x + " Celsius";

Function Input and Output

The most useful functions work like this:

• Parameters - The names listed in the function definition
• Arguments - The values sent to the function when it is called
• Function Code - The work done inside the function
• Return Output - The value returned from the function

Example

function greet(name) {

  return "Hello " + name;

}

greet("Riya");

In the example above: