A FUNCTION is a NAME for a piece of code

function greetByName(someName) {
  return 'Hello there! It is nice to meet you ' + someName + '.';

This is similar to how a variable is a name for a piece of data

Why Name a Piece of Code?

Here are some reasons why this is useful:

  • Use code from several places in a file
  • Perform the same procedure on different input values
  • Organize your code into different 'modules' of functionality
  • Limit visibility and access of variables, or values

Function example

Here's an example function:

function add(firstNum, secondNum) {
  let sum = firstNum + secondNum;
  return sum;
  • function means, define a function
  • add is the name of the function
  • firstNum, secondNum are the parameters to the function, also called arguments
  • sum is a local variable of the function
  • sum is also the return value of the function

Calling a Function

You call a function by referencing the name followed by parentheses:

function add(firstNum, secondNum) {
  let sum = firstNum + secondNum;
  return sum;

add(2, 3)   // returns 5
add(12, 30) // returns 42

This is also known as executing the function.

Lab: One Nation, Divisible

A number is divisible by another if when you divide them, the remainder is 0.

Write a function called divisible that:

  • Accepts two numbers as parameters
  • Returns true if the first number is divisible by the second number, and false otherwise
function divisible(firstNum, secondNum) {
  // write your code here

divisible(100, 10)  // true
divisible(100, 7)   // false
divisible(3333, 11) // true
divisible(99, 12)   // false

If you write the solution in a file, use console.log(divisible(100, 7)) to print the return value.

Divisible solution

Hint 1
The modulus operator `%` returns either an integer or zero. Integers are truthy, while 0 is falsy.
Hint 2
You may need to use some logic. You can create a simple control flow with an `if ...else` statement
  if(/*this expression evaluates true*/) {
    //do this
  } else {
    //otherwise do this
  function divisible(operator, operand) {
    if(operator % operand) {
      return true
    } else {
      return false

  divisible(100, 10) // => true


Here is a function that takes a String as input, and it returns a shouted version of that String.

function shouter(someString) {
  let loudString = someString.toUpperCase();
  return loudString + '!!!';

shouter('i like pizza');
// 'I LIKE PIZZA!!!'

The variable loudString is called a local variable and can only be used inside the function.

Lab: Capitalize

Write a function called capitalize that:

  • Accepts a one word string as a parameter.
  • Returns a new Capitalized version of a word.
  • Remember to only uppercase the first letter of the word.
function capitalize(someWord) {
  // your code here

// 'Tomato'

Remember there are many string operations

Capitalize Lab Solution

Hint One
Try getting the first letter from the String using indexing
let firstLetter = string[0]
Hint Two
Try using the method Slice
let restOfString = string.slice(1)

    function capitalize(word) {
      let firstLetter = word[0];
      let restOfWord = word.slice(1);
      return firstLetter.toUpperCase() + restOfWord.toLowerCase();

The variables `firstLetter` and `restOfWord` are called *local variables* and can only be used *inside* the `capitalize` function.

Passing Variables as Parameters

When you pass a variable to a function, that variable value is assigned to a parameter.

let nameToShout = 'Grace Hopper';


The variable name and parameter name DO NOT need to match

Shouter Function Explanation

let nameToShout = 'Grace Hopper';

function shouter(someString) {
  let loudString = someString.toUpperCase();
  return loudString + '!!!';

let result = shouter(nameToShout);
Outside Inside Value
nameToShout someString 'Grace Hopper'
loudString 'GRACE HOPPER'
result 'GRACE HOPPER!!!'

Lab: Age Calculator

Write a function named ageInSeconds that:

  • Accepts a person's age in years
  • Multiplies the age in years by the number of seconds in a year
  • returns the person's age in seconds
let age = 27;

function ageInSeconds(num) {
  // your code here

// 'You are 852055200 seconds old'

Age Function Reverse

Now write a reverse function that:

  • Accepts a number of seconds as a parameter
  • Divides the age in seconds by the number of seconds in a year
  • returns the age in years, or fractions of a year
let ageInSeconds = 852055200;

function ageInYears(seconds) {
  // your code here

// 'You are 27 years old'


Here's one solution for the ageInSeconds calculator:


  let age = 27

  function ageInSeconds(num) {
    let secondsInMin = 60
    let minInHour = 60
    let hrInDay = 24
    let dayInYr = 365.25

    let secInYr = secondsInMin * minInHour * hrInDay * dayInYr

    let ageInSec = num * secInYr

    return ageInSec


Lab: Supply Calculator

Write a function named supplyCalc that:

  • Accepts three arguments, a starting age, an amount per day, and an item name
  • Calculates the amount of items used over the rest of your life, based on a 100 year max age
  • returns 'You will need Number Items to last the rest of your life.' e.g.
supplyCalc(20, 3, 'cookie') 
// 'You will need 87600 cookies to last the rest of your life'
supplyCalc(99, 3, 'cakes')
// 'You will need 1095 cakes to last the rest of your life'
supplyCalc(0, 3, 'pies')
// 'You will need 109500 pies to last the rest of your life'

Inspired by the Lifetime Supply Calculator lab designed for the Girl Develope It! curriculum. The original can be found here

Supply Calculator Solution

Hint 1 Calculate the number of cookies needed per year

let amountPerYear = amountPerDay * 365
Hint 2 Calculate the number of years of life left

let numberOfYears = 100 - age
Solution Use the number of cookies per year, and number of years of life, to calculate the total cookies

function supplyCalc(age, amountPerDay, item) {
  let amountPerYear = amountPerDay *365;
  let numberOfYears = 100 - age;
  let totalNeeded = amountPerYear* numberOfYears;

  return 'You will need' + totalNeeded + ' ' + item + 's to last the rest of your life';

Lab: Titleize

Write a function named titleize that:

  • Accepts a string as an argument
  • Splits apart the words in the string
  • Capitalizes each word
  • returns a string with the first letter of each word capitalized e.g.
titleize('all dogs are good dogs');
// 'All Dogs Are Good Dogs'
titleize('eveRY green bus drives fAst');
// 'Every Green Bus Drives Fast'
// 'Friday Is The Longest Day'

Titleize solution

Hint 1 Make a function to capitalize each word

function capitalize(word) {
  let firstLetter = word[0].toUpperCase();
  let restOfWord = word.slice(1).toLowerCase();
  return firstLetter + restOfWord;
Hint 2 Split the words in the string into an Array of items

let wordArray = string.split(' ');
Solution Loop over every item in the Array of words, capitalize each, and then add them to a new string

function capitalize(word) {
  let firstLetter = word[0].toUpperCase();
  let restOfWord = word.slice(1).toLowerCase();
  return firstLetter + restOfWord;

function titleize(string) {
  let wordArray = string.split(' ');

  let newString = '';
  let wordsModified = 0;

  while (wordsModified < wordArray.length) {
    let currentWord = wordArray[wordsModified];
    let newWord = capitalize(currentWord);
    newString = newString + ' ' + newWord;
    wordsModified = wordsModified + 1;

  return newString.trim();

More About Functions