Arrays are Objects


Every JavaScript array is also a JavaScript object

That means that arrays have properties and methods like any other object.


  • array.length is a read-only property that always contains the number of elements in the array
  • array.reverse() is a method that reverses the ordering of all elements in the array

See MDN: Array to see a lot more array methods

Iteration Methods

Every JavaScript array has a few very handy methods that let you apply a function to its contents.

method description returns
forEach do something to each item undefined
find find the first item that matches one matching item (or undefined if no match)
filter accept or reject each item a new collection, possibly smaller
map change each item into a new item a new collection of the same size
reduce scan the entire collection and "reduce" it to... ...a single result, e.g. a total
  • We call this group of methods "iteration methods"
  • There are about a dozen built-in iteration methods, plus lots more added by libraries like lodash.


forEach works a lot like for..of

Given an array of names:

let names = ['Alice', 'Bob', 'Charlie', 'Carol', 'David']

this code:

for (let name of names) {

and this code:

let printUpper = function(word) {

both print:



to find the first item that matches the condition...

let names = ['Alice', 'Bob', 'Charlie', 'Carol', 'David']
let beginsWithC = function(word) {
    return word.charAt(0).toUpperCase() === 'C';
names.find(beginsWithC) //=> 'Charlie'

Find Inline

For conciseness, people often define the filter function inline, like this:

names.find((word) => word.charAt(0).toUpperCase() === 'C')

Q: Is this more or less clear than the previous slide?

Lab: Find a Berry

Given the following array:

let fruits = ['Apple', 'Blueberry', 'Cherry']

write some code that uses find to return the first item that ends with the string 'berry'


let names = ['Alice', 'Bob', 'Charlie', 'Carol', 'David']
let beginsWithC = function(word) {
    return word.charAt(0).toUpperCase() === 'C';
names.filter(beginsWithC) //=> [ 'Charlie', 'Carol' ]

Lab: Find all Berries

Given the following array:

let fruits = ['Apple', 'Blueberry', 'Cherry', 'Date', 'Elderberry']

write some code that uses filter to return a new array containing all the fruits that end with the string 'berry'


let names = ['Alice', 'Bob', 'Charlie', 'Carol', 'David']
let upper = function(word) {
    return word.toUpperCase();
} //=> [ 'ALICE', 'BOB', 'CHARLIE', 'CAROL', 'DAVID' ]

Lab: Titleize with Map

Remember capitalize? It capitalizes the first letter of a string and makes the whole rest of the string lowercase.

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

Now please try to write a function that capitalizes each word in a string.

titleize("the rain in spain falls MAINLY on the PLAIN")
  //=> 'The Rain In Spain Falls Mainly On The Plain'

There is a solution on the next slide, but please try on your own first.

Hint: inside your titleize function, you should use the existing capitalize function

Solution: Titleize

Here's one way to do it:

function titleize(phrase) {
  return phrase.split(' ').map((word) => capitalize(word)).join(' ');

Here's another:

function titleize(phrase) {
  return phrase.split(' ').map(capitalize).join(' ');

And another:

function titleize(phrase) {
    let words = [];
    phrase.split(' ').forEach((word) => {words.push(capitalize(word))});
    return words.join(' ');


The reduce method keeps track of a running total (aka accumulator); whatever value the function returns is used as the accumulator for the next pass.

Here's some code that counts the total number of letters across all words in an array:

let names = ['Alice', 'Bob', 'Charlie', 'Carol', 'David'];
const reducer = (accumulator, word) => accumulator + word.length;
let totalCount = names.reduce(reducer, 0); //=> 25

The reduce algorithm can be difficult; here's a walkthrough:

Iteration Accumulator In Word Length Accumulator Out
1 0 'Alice' 5 0 + 5 = 5
2 5 'Bob' 3 5 + 3 = 8
3 8 'Charlie' 7 8 + 7 = 15
4 15 'Carol' 5 15 + 5 = 20
5 20 'David' 5 20 + 5 = 25

See how the accumulator is used to pass information from one iteration to the next?

Map Filter Reduce in Emoji

map filter reduce in emoji

(source unknown)