Software Models the World

  • Data Structures allow for programmers to make models of the world in code.
  • People naturally think of the world as made up of things.
  • Programming languages have ways to represent things.
  • JavaScript's primary way of representing a thing is an "Object".
  • JavaScript objects let you describe things in any way you can imagine.
    • As long as your description is valid JavaScript :)

All models are wrong, but some are useful. -- George Box

Objects Contain Properties

let fido = {
  species: 'dog',
  color: 'brown',
  spayed: true,
  weight: 40,
  favoriteActivities: [
    'chasing squirrels'
  • Object properties are KEY and VALUE pairs
  • The key is on the left and the value on the right; the two together are called a property.
  • JavaScript object keys are always strings. If the key has no spaces you can omit surrounding quotes.
  • let fido = {...} is an assignment, setting the variable fido to point to the object we just defined

An Object is like a Lookup Table

Objects are useful for putting many similar values together.

Let's make an object that binds a state abbreviation to its full name

Type this in a NodeJS console, or a JavaScript file:

let states = {
  'VT': 'Vermont',
  'CA': 'California',
  'MA': 'Massachusetts',
  'NY': 'New York'

Getting Object Properties

You can get the properties of an object using either dots or brackets:

With Dots With Brackets The Value
states.VT states['VT'] 'Vermont'
states.CA states['CA'] 'California'
states.MA states['MA'] 'Massachusetts'
states.NY states['NY'] 'New York'

Both syntaxes are useful in different situations.

states['VT']  // 'Vermont'
states.VT     // also 'Vermont'

Setting Object Properties

  • You can set the properties of an object with either dots or brackets followed by = ( a single equal sign)
  • Adding properties works even after the object has been created.
states.WY = 'Wyoming'

states['FL'] = 'Florida'
  • Setting a property with a key that already exists updates the original value javascript states.VT = 'The Green Mountain State'

Dots vs. Brackets 1

Dots are prettier than square brackets, but less versatile, since some keys simply cannot be represented using dot notation, and trying to use them causes syntax errors.

The bracket [] syntax is less common but covers all uses (e.g., if the key contains spaces, or is a variable).

> let capitals = {}

> capitals.New York = 'Albany'
capitals.New York = 'Albany'
SyntaxError: Unexpected identifier

> capitals.'New York' = 'Albany'
capitals.'New York' = 'Albany'
SyntaxError: Unexpected string

Dots vs. Brackets 2

If you get an error when setting a property, revert to brackets, which are more reliable:

> capitals['New York'] = 'Albany'
> capitals
{ 'New York': 'Albany' }

Dots vs. Brackets with Variables 1

You can use variables instead of values to get and set properties.

Given this code ...

let materials = {
  brick: 'a red brick',
  siding: 'some wooden clapboards'

let desired = 'brick'
// => 'a red brick'

Dots vs. Brackets with Variables 2

The last two expressions look for a key named desired, but only the first one looks for a key named the value of the variable named desired:

code value explanation
materials[desired] 'a red brick' "get me the property named 'brick'"
materials.desired undefined "get me the property named 'desired'"
materials['desired'] undefined "get me the property named 'desired'"

NOTE: This can be confusing!

An Object is a Data Structure

Objects are good for a lot more than mere one-to-one maps. They allow you to design data structures that are as complicated and as deeply nested as you can imagine...

let alice = {
  name: 'Alice Abrams',
  age: 36,
  married: false,
  homeAddress: {
    street: '12 Maple St.',
    city: 'Burlington',
    state: 'VT',
    zipCode: '05401',
    location: {
      latitude: 44.4759,
      longitude: -73.2121
  pets: ['Mittens', 'Fido']

// => '05401'

Given the above, the value of alice.homeAddress.zipCode is '05401'

LAB: Looping through an object with

There is a way to loop through every property in an object.

This loop:

  • Grabs every property in the states object, one at a time
  • And inside the loop
  • Sets the state value to that property's key
for (let state in states) {
// write code here 
// using the state variable
// and the states object

In a new JS file, try to write code that outputs:

VT is short for Vermont
CA is short for California
MA is short for Massachusetts
NY is short for New York

let states = {
  'VT': 'Vermont',
  'CA': 'California',
  'MA': 'Massachusetts',
  'NY': 'New York'

for (let state in states) {
    console.log(state + ' is short for ' + states[state]);

NOTE: Use for objects, and not for..of. See this article for more detail about of vs. in.

WARNING: remember to use the let keyword or you will be defining a global variable named state

Object Properties as an Array

It is also possible to convert an Object's properties into an Array of Strings, and then loop over that Array

Object.keys(someObject) is a function that

  • Receives any object as an argument
  • Returns an array of all the object's property names


let states = {
  CA: 'California',
  MA: 'Massachusetts',
  NY: 'New York'

//=> [ 'CA', 'MA', 'NY' ]

MDN Documentation on Object.keys

LAB: Class GPA

Create a new JavaScript file called gpa.js, with the following code:

let grades = {
  midterm: 3.3,
  project: 4.0,
  final: 3.2

function gpa(someGrades) {
  // write your code here

console.log('The final GPA is ' + gpa(grades));
  • Write the gpa function
  • Assume that all grades carry equal weight toward the GPA

The answer is 3.5 ... but don't just return 3.5, make it work for real. There is more than one way to solve this.

Class GPA solution

Hint 1
You can use `` to loop over an object's properties
Hint 2
You can use `Object.keys(someObject) to get all an object's properties as an Array, then check their length with `.length`

let grades = {
  midterm: 3.3,
  project: 4.0,
  final: 3.2

function gpa(gradeObject) {
  let total = 0;
  let gradeCount = Object.keys(gradeObject).count;
  for (let grade in gradeObject) {
    total = total + gradeObject[grade];
  return total / gradeCount;

console.log('The GPA is ' + gpa(grades));

All keys become Strings

  • In a JavaScript object, keys must be strings, even null, 0, and undefined

  • Beware of using these as keys, since they get converted to strings in unexpected ways:

    • null
    • undefined
    • '' (empty string)
    • false or true
    • 0 (or any number)

The delete Operator

To remove a key-value pair from an object, use the keyword delete:

let states = {
  CA: 'California',
  MA: 'Massachusetts',
  NY: 'New York'

// => { CA: 'California', MA: 'Massachusetts', NY: 'New York' }

delete states.MA

// => { CA: 'California', NY: 'New York' }

NOTE: delete is not a function, it is an operator

A Way to Fake delete

You can get a similar effect by setting the value to null or undefined, but beware: the key remains!

let states = {
  CA: 'California',
  MA: 'Massachusetts',
  NY: 'New York'

states.CA = null
states.NY = undefined
// => { CA: null, NY: undefined }

for (let state in states) { 
// => null
// => undefined

NOTE: This only changes the value, but not the key

LAB: A Menu Order

Write a function that accepts an order with items in a menu object

  • Name your file order.js
  • Create an object, called menu, to store all the item names and their prices
  • Accept an order as an Array of Strings, containing the desired items
  • Print the total price, based on what is ordered
ItemName Price
burger $5.00
fries $3.50
shake $1.11
salad $4.25

Object Methods Preview

Here is a little preview of object methods

A METHOD is any function attached to an object as a property

The object can then call that property as a function

let stringHelpers = {
  capitalize: function(word) {
    return word.charAt(0).toUpperCase() +
  yell: function(opinion) {
    return opinion.toUpperCase() + '!!!';

stringHelpers.yell('i love pizza') //=> 'I LOVE PIZZA!!!'

More About JS Objects

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