Functions - Reusable Blocks of Code

What Is a Function?

A function is a block of instructions in your code that you can use over and over again. You give the function a name, and when you call that name, the code inside the function will execute. Functions can take inputs (we call these parameters) and can give back an output (called a return value).

Declaring a Function

To create a function, you can use the function keyword. For example, this function prints a greeting:

1function sayHello() {
2  console.log("Hello, Function!");
3}

Here, we have:

• Declaration: The keyword function tells JavaScript we are creating a function.
• Name: The function is named sayHello.
• Body: The code inside the curly braces {} is what the function does. In this case, it prints a message.

Calling a Function

Once a function is declared, you can run it (or “call” it) by writing its name followed by parentheses:

1sayHello();

This tells JavaScript, “Please run the instructions inside the function called sayHello.”

Functions with Parameters and Return Values

Functions can also accept inputs. For example, here’s a function that adds two numbers:

1function addNumbers(a, b) {
2  return a + b;
3}

In this case:

• Parameters: The function takes two inputs, a and b.
• Return Value: The function returns the result of adding a and b.

You can call this function and store its result in a variable:

1let sum = addNumbers(3, 4);
2console.log(sum); // prints 7

Exercises