Function definition

The name, arguments, and code for a function.

function doSomething(){}

A function lets you create a portion of code that you can reuse in your program. This is handy because you can put code that you want to use over again in a function. Instead of copying the same code to many places in your program, you can just use a function you made and all the code inside is used as a single block.

The function word

All functions definitions begin with the word function. This means that the definition will start.

function myFunction() {


A function has a name that is unique. You also give a name to a function which gives an idea about what the code in the function does. If you want to make a function to set all LEDs to green, then you might call the function, setAllGreen. This gives you and anyone else looking at your code an idea of what the function will do.

function setAllGreen() {
    let lightsOn = true;

If you had some functions with the names function1, function2, and function3, they would be unique but you probably wouldn’t remember what each one does the next time you looked at your code. That’s why it’s best to give your functions names that tell you what the code inside is going to accomplish.


After you make the name of the function, you add the argument list. In JavaScript, the argument list is inside two ( ) braces. Right now, your functions don’t use any arguments.


The body of the function is the code inside of the function block. In JavaScript, it is what goes between braces { } of the function. The work of the function happens here.

let side = 25;
let square = 0;

function squared( ) {
    square = side * side;


Make a function to calculate the average of score of quiz scores in a list.

let scores = [9, 5, 3, 1, 6, 3, 5, 7];
let average = 0;

function averageScore( ) {
    let count = 0;
    for (let score of scores) {
        count += score;
    average = count / scores.length;

See also