Remember that a variable is a name for a piece of data

A function is the name for a piece of code

Why would you want to name a chunk of code?


  • you have some code you want to run again and again
  • you want to do the same operation on different values
  • you want to keep your code organized

Function example

Here's an example function:

function add(x, y) {
  let sum = x + y;
  return sum;
  • function means "define a function"
  • add is the name of the function
  • x, y are the parameters of the function (also called arguments)
  • sum is a local variable of the function
  • sum is also the function's return value because of the magic word return

Calling a Function by Name

You call a function by its name, plus parentheses:

function add(x, y) {
  let sum = x + y;
  return sum;

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

Lab: One Nation, Divisible

One number is divisible by another if you divide them and the remainder is 0.

Write a function called divisible that takes two numbers, and returns true if the first number is divisible by the second number, and false otherwise.

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

Divisible solution

Hint 1
The modulus operator `%` returns either an integer or zero. Integers are truthy, while 0 is falsey
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 some String as input, and as output 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

Please write a function that returns a Capitalized version of a word. Only uppercase the first letter of the word.

For example:

capitalize('tomato') returns 'Tomato'

A solution is on the next slide, but try to solve it on your own. Remember that there are many string operations available to you...

Capitalize Lab Solution

Useful String Methods

String Manipulation

Hint One Try getting the first letter from the String using indexing
```js let firstLetter = string[0] ```
Hint Two Try using the method `slice` [String Slice - Mozilla](
```js let restOfString = string.slice(1) ```
function capitalize(word) { let firstLetter = word[0]; let restOfWord = word.slice(1); return firstLetter.toUpperCase() + restOfWord.toLowerCase(); } console.log(capitalize('smith')); console.log(capitalize('MACGUYVER')); The variables `firstLetter` and `restOfWord` are called *local variables* and can only be used *inside* the `capitalize` function.

Passing Variables to Functions

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

The variable and parameter names do not need to match!

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

let feeling = "I feel great";
let strongFeeling = shouter(feeling);
Outside the function Inside the function Value
feeling someString "I feel great"
loudString "I FEEL GREAT"
strongFeeling "I FEEL GREAT!!!"

Four Function Syntaxes

WARNING: JavaScript has many ways to define a function.

Function declaration syntax

function add(x,y) { return x + y; }

The following are all roughly equivalent to the above:

Function Expression

let add = function(x,y) { return x + y; };

Arrow Function Expression

let add = (x,y) => { return x + y; };

Arrow Function Expression with implicit return value

let add = (x,y) => x + y;
  • Note that these new forms are anonymous, meaning there is no name between function and (x,y)
    • the name of the function is the name of the variable that points to it

Lab: Age Calculator

Write a function that calculates the number of seconds old you are when given your age

let age = 27

function ageCalc (num) {
  //Your code goes here

ageCalc(age) // should print "You are 852055200 seconds old." to the console

How could we use ARGV to make this more modular?

Flip it around!

Can you write the inverse function; one that takes a number of seconds and tells you the exact age?

You can get the current date by calling which will give you a time in milliseconds, and the date you were born by creating a new Date object. You can then figure out the time that's elapsed in milliseconds by subtracting the date you were born from the current date

  let date = new Date(1992, 05, 12, 3, 14) //new Date(year, month, day, hour, minute)
  let ageInMilliSec = - date


Here's one solution for the age calculator:

let age = 27 function ageCalc(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 } console.log(ageCalc(age)) To flip it you could simply divide the `num` variable by `secInYr` rather than multiplying to get years in a number of seconds.

Lab: Supply Calculator

Write a function that:

  • accepts three arguments, a starting age, an amount per day, and an item name
  • calculates the amount of items used over the course of the rest of your life
    • based on a 100 year constant max age
  • Outputs "You will need Number Items to last the rest of your life." e.g.
supplyCal(20, 3, "cookie") // => "You will need 87600 cookies to last the rest of your life"
supplyCal(99, 3, "cookie") // => "You will need 1095 cookies to last the rest of your life"
supplyCal(0, 3, "cookie") // => "You will need 109500 cookies to last the rest of your life"

Supply Calculator 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
let amountPerYear = amountPerDay * 365
Hint 2
let numberOfYears = 100 - age
function supplyCalc(age, amountPerDay, item) { let amountPerYear = amountPerDay * 365 let numberOfYears = 100 - age let totalNeeded = amountPerYear * numberOfYears let message = "You will need" + totalNeeded + " " + item + "s to last the rest of your life" }

Lab: Titleize

Write a function 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.
titilize("all dogs are good dogs") // => "All Dogs Are Good Dogs"
titilize("eveRY green bus drives fAst") // => "Every Green Bus Drives Fast"
titilize("FRIDAY IS THE LONGEST DAY") // => "Friday Is The Longest Day"

Titilize solution

Hint 1
function capitalize(word) { return word[0].toUpperCase() + word.slice(1).toLowerCase() }
Hint 2
let wordArray = string.split(" ")
function titilize(string){ let wordArray = string.split(" ") let newArray ={ return word[0].toUpperCase() + word.slice(1).toLowerCase() }) return newArray.join(" ") }

More About Functions