• an object that contains other objects
  • It's a list of objects

What makes an array an array

  • You can put any values inside it
  • In any order
  • They stay in order (unless you move them later)
  • Duplicates are fine

Creating an array

["apple", "banana", "cherry"]

square brackets on their own mean "please go create an array now"

and put these 3 other values inside it

Array Indexes

  • Every slot in the array has a serial number
  • You can retrieve any item in an array by its INDEX
  • Square brackets after an array mean "the whatever-th item in this array"
  • The following code retrieves one fruit
let fruits = ["apple", "banana", "cherry"];

...but which fruit? See next slide!

LAB: Array Indexes Exercise

Try this in node:

let fruits = ["apple", "banana", "cherry"]

Did you get the result you expected?

Why or why not?

Start At Zero

When counting,

humans start at one,

but computers start at zero.

So the first item in an array is number zero, not number one.


Every array has a property named length

let fruits = ["apple", "banana", "cherry"]
fruits.length //=> 3

Q: How can you get the last item in an array... even if you don't know its index beforehand?

The End

let fruits = ["apple", "banana", "cherry"]
fruits[fruits.length - 1]

After The End

Try this:


Did you get the result you expected?

Why or why not?

Undefined means 🤷

by returning undefined, the computer is answering the question

"What is the 99th item?"

with the answer

"There is no 99th item."

Array Methods

MDN: Array lists the array API interface -- containing all the methods and properties that are common to all array values.

There are many methods here and you should not try to memorize them all. But skim them and remember how to get back to this documentation page later.

Adding values to an array

  • push adds a single value to the end of an array
let fruits = ["apple", "banana", "cherry"]
fruits //=> ["apple", "banana", "cherry", "pineapple"]
  • push can also add several values at once
fruits.push("nectarine", "strawberry")
fruits //=> ["apple", "banana", "cherry", "pineapple", "nectarine", "strawberry"]

Yarra Lasrever

let fruits = ["apple", "banana", "cherry"]

Try this now in a console. Do you see what you expect?

Slicing and Dicing

you can slice an array to cut it into smaller arrays

let fruits = ['apple', 'banana', 'cherry', 'date', 'elderberry']

// this means "slice from item 1 to item 3"
fruits.slice(1, 3) //=> [ 'banana', 'cherry' ]

// this means "slice from item 2 to the end"
fruits.slice(2) //=> [ 'cherry', 'date', 'elderberry' ]

These start and end numbers are called indexes (or indices if you're feeling fancy).

MDN: slice

Array Indexing Explained

Humans like to start counting at 1, but computers like to start counting at 0.

This can be confusing, so here's a visualization to help explain it.

Think of the indexes as pointing at the spaces between items, as in this diagram:

['B', 'L', 'U', 'E']

| B | L | U | E |
0   1   2   3   4

So with this picture in your mind, imagine that slice...

  • includes the item to the right of the start index
  • includes the item to the left of the end index...
  • ...but excludes the item to the right of the end index
['B', 'L', 'U', 'E'].slice(1, 3) //=> [ 'L', 'U' ]

Array to String

There are a few easy ways to turn an array into a string.

fruits.join()           // 'apple,banana,cherry'
fruits.join(" and ")    // 'apple and banana and cherry'
fruits.toString()       // 'apple,banana,cherry'
fruits.toSource()       // [ 'apple', 'banana', 'cherry' ]

Note that console.log uses toSource(), which is usually better for debugging than toString...

...but the toSource method only works in web browsers, not in NodeJS, at least not by default :-(

String to Array

You can also easily turn a string into an array.

'dog'.split('') //=> ['d', 'o', 'g']
'my dog has fleas'.split(' ') //=> [ 'my', 'dog', 'has', 'fleas' ]

Loops and Iterators

There are many ways to "iterate" through an array.

This means to go through the entire array, one item at a time, usually in order, and then do something with each individual item.

In the next slides we will illustrate 3 different ways to iterate... one way is explicit, one way is concise, and one way is fancy.

Explicit: Looping through an array with for

JavaScript inherited for(;;) from C; it's cumbersome but you should learn to recognize it.

for (let i=0; i < fruits.length; i++) {
phrase meaning
for in a loop,
let i make an index variable named i
i=0 and initially set it to 0
i < fruits.length then, as long as i is less than the number of fruits
{ ... } execute this block of code
console.log(fruits[i]) print the ith element of the fruits array
i++ and then increment i before the next time through

Concise: Looping through an array with for-of

Recently, JavaScript added for..of, which hides the messy details of incrementing an index counter and accessing each array item.

for (let fruit of fruits) {
  console.log("I like " + fruit + "!")
phrase meaning
for in a loop,
of fruits take each thing inside fruits
let fruit name it fruit
{ ... } and send it to this block of code

Fancy: Looping through an array with forEach

forEach is an iteration method that behaves a lot like for..of but in a functional style :

fruits.forEach( (fruit) => {
  console.log("I like " + fruit + "!")
phrase meaning
fruits.forEach(...) hey fruits, for each thing inside you,
(fruit) please name it fruit
=> and send it to
{ ... } this block of code
console.log(fruit) so I can print it to the terminal

MDN: Array.forEach

LAB: loud backwards fruit

Given this array:

let fruits = ["apple", "banana", "cherry"]

write a program that prints:


Setting items in an array

The [] operator works for assignment as well.

fruits[0] = 'apricot' will set the 0th item of the array to the string 'apricot'

Checking every item in an array

The includes method checks if a given value is inside an array (or not).

fruits.includes("apple")   // true

fruits.includes("pizza")   // false

Remember, here we are sending a message to an array, asking if it includes a certain string.

LAB: enemies list refactoring

Refactoring is changing existing code so that it works the same, but is cleaner and easier to read.

In your old hello.js program you had an if statement to check if someone is your enemy. Something like:

if (name === "darth" || name === "voldemort" || name === "sauron")

Please take a few minutes to refactor your old hello.js program to use arrays to check if someone is your enemy. * Put the list of enemies in an array * How can you use array methods to check if a name is in the enemies list? * BEWARE: your new code might need more curly braces than the old code; make sure the {s and }s properly match

LAB: more about arrays