Objects Lesson

Lesson Objectives

After this lesson, students will be able to:

  1. Explain the difference between arrays and objects

  2. Store key-value pairs inside an object

  3. Access values by key-name

  4. Add Object Properties

  5. Change Object Properties

  6. Explain why we use an object instead of an array

  7. Manipulate objects and arrays declared as const

  8. List common errors made with objects

  9. Use object properties with conditionals

Explain the difference between arrays and objects

We have seen the following datatypes:

  • String

  • Number

  • Boolean

  • Arrays

Arrays are a data structure. We use them to organize our data: in the case of arrays, we can organize our data into a sequential list data structure.

  • We can use arrays to store multiple pieces of data as a sequential list:

const vehicle = ["blue", 4000, 1989];

  • Each element has a corresponding index (or place), in sequence.

But with the array above, we don't know what the values mean. Does "blue" refer to the color of the vehicle? To the mood of the owner? Is it the model of the vehicle?

  • An object is also a data structure, but we can use objects to store data with greater specificity.

  • In JavaScript, objects are what we use to represent key-value pairs.

Store key-value pairs inside an object

Key-value pair syntax:

const car = {
    color: "blue",
    hp: 4000,
    year: 1989
}

  • Unlike arrays, objects use named keys rather than ordered indexes. Each piece of data is bound to its key, rather than assigned an index. The data is not sequential.

  • In Javascript, an object is a way to group many pairs of keys and values together

We can console.log the entire object:

console.log(car);

Access values by key-name

We can access the values stored in key using dot notation:

console.log(car.color)

Question: Ask (5 min)

  • What differences do you see between the vehicle array and the car object syntax?

const vehicle = ["blue", 4000, 1989];
const car = { color: "blue", hp: 4000, year: 1989 };

Differences between arrays and objects

  • Arrays are declared using the square brackets var arr = [];

  • Objects are declared using the curly braces var obj = {}

Objects contain key-value pairs. They are are the properties of the object

A key is like an index in an array, but it has

  • a name

  • it is unique

A key is really a string but we can omit the quotes.

A value is what a key refers to, and can be any datatype.

Class Exercise: Build an Object

Build an object in detail, demonstrating that:

  • We use a colon to separate the key and the value

  • We do not put semicolons after our values.

  • We separate our key-value pairs with a comma

const person = {}

Investigate

  1. What properties should we add to the person object.

  2. How can we access these properties.

Activity (10 min)

Dog

  • Create an object called dog that has the following properties:

    • name (a string, give your dog a name)

    • age (a number, give your dog an age)

    • Remember the correct use of curly braces, colons, and commas! No semicolons!

  • Console.log the dog object to check if it's correct

  • Console.log just the dog's name

  • Console.log just the dog's age

Celebrity

  • Create an object called celebrity that has the following properties:

    • name (a string, give the celebrity a name)

    • age (a number, give the celebrity an age)

    • isCurrrentlyTweeting (a boolean)

  • Console.log the celebrity object

  • Console.log just the name of the celebrity

  • Console.log just the age of the celebrity

  • Console.log whether or not the celebrity is currently tweeting

  • Write conditional that will print "Turn off Twitter" if the celebrity is currently tweeting.

Add Object Properties

You can easily add more properties to a previously declared object:

const house = {
    doors: 9
}

console.log(house)

=> { doors: 9 }

Add properties to the house object by simply adding a key using dot notation and the value using an equals =. Our house has no windows. Let's add some in without writing them straight into the object:

house.windows = 30

When we do it this way, the windows key is added to the object.

console.log(house);

=> { doors: 9, windows: 30 }

Add another property hasGarden:

house.hasGarden = true;

Change Object Properties

Changing the value of an existing key has the same syntax as creating a new key-value pair:

const bicycle = {
    isATricycle: false
}
bicycle.isATricycle = true

Activity (7 min)

  • Create an empty object called macros

Do not write in to this empty object. Instead, add keys and values with macros.keyName = "value" After each key-value addition, console.log the macros object to make sure the new keys and values show up.

  • Add to the object a key protein with a value 'tempeh'

  • Add to the object a key carbohydrates with a value 'spuds'

  • Add to the object a key fats with a value 'olive oil'

  • Console.log the macros object to check if all the macros are there

Activity: 10 minutes

  • Create an object called guitar with the following properties:

    • a key strings with value 6

    • a key isAcoustic with value true (boolean)

  • Change the value of strings to 100

    • console.log the value of strings (guitar.strings)

  • Change the value of isAcoustic to false

    • Console.log the value of isAcoustic

  • Without writing into your object directly, add a key belongsTo with the value 'Dimebag Darrell'

    • Console.log the value of belongsTo

  • Change the value of belongsTo to 'Mr. Rogers'

  • Console.log the entire guitar object

Explain why we use an object instead of an array

When designing your programs, it is up to you to choose how to model your data. We can represent real-life things with our datatypes, but it's a matter of choosing the appropriate datatypes.

If the thing you want to model is just a list, use an array.

If the thing want to model has properties, use an object.

Using what we know about datatypes so far, which datatype would we use to model:

  1. The name of your cat

  2. The age of your cat

  3. Your cat's favorite things

  4. Whether your cat can speak French

  5. Whether your cat can solve a Rubik's cube

  6. Your cat

Manipulate objects and arrays declared as const

const only prevents you from reassigning a variable, it doesn't prevent you from adding or changing elements of arrays or properties of objects.

You can do this:

const mogwai  = {}

mogwai.name = 'Gizmo';

Cannot do this:

const mogwai = {}

mogwai = { name: 'Gizmo' }

Object literal shorthand

If variable names outside the object will correspond to the keys in the object, you can write shorthand like this for the mogwai object:

const name = 'Gizmo';
const age = 1;

const mogwai = { name, age }

console.log(mogwai);

=> { name: 'Gizmo', age: 1 }

This is equivalent to the longhand:

const name = 'Gizmo';
const age = 1;

const mogwai = { name: name, age: age }

console.log(mogwai);

=> { name: 'Gizmo', age: 1 }

List common errors made with objects

Unique Keys

It just makes sense that keys ought to be unique within an object. Values, however, can be whatever.

An object can not have more than one key with the same name. If it does, the value will default to the last key with the same name, and the prior properties will be excluded on creation.

const borough = {
    name: "Brooklyn",
    name: "The Bronx"
}
console.log(borough);

=> Object { name: "The Bronx" }

Conclusion: keys should be unique within an object. Values, however, are not unique.

Accessing and Naming Keys Using Brackets and Quotes

You can create and access any key with square brackets and quotes.

const goblin = { badGuy: true };
console.log(goblin['badGuy']);
=> true

With square brackets and quotes, you can make key names with spaces and special characters, because the key is coerced into a string. But you then have to access the value from here on out with square brackets and quotes.

const strangeObj = {}

strangeObj['a key with spaces'] = 999;

console.log(strangeObj)
=> Object { 'a key with spaces': 999 }

You would need also to access that key with the square brackets and quotes:

console.log(strangeObj['a key with spaces']);

=> 999

You could not access that key using dot notation.

Square brackets are nice if you need to programmatically generate a key name:

const obj = {};
for (var i = 0; i < 10; i++) {
    obj['key'+i] = 'foo'
}
console.log(obj);

Keys That Are Numbers

If a key is just a number, that number will be coerced into a string, which is fine.

const obj = {
    1: "one",
}
console.log(obj);
=> Object { '1': 'one' }

But, you cannot access, add, or change numbered keys with dot notation.

console.log(obj.1)
obj.2 = "hey"

console.log(obj2);

There is another way to access key-values using square brackets and quotes obj['1']

Activity

  • Create an empty object called testObject

  • Give testObject a key called 'this is a test' with the value "test"

    • Console.log the value of the key 'this is a test'

  • Give test object a key called 2 with the value "I'm just messing around with objects"

    • Console.log the value of the key 2

Use object properties with conditionals

You can use object properties with conditionals, loops, etc

const obj = {
    whatevs: 'hi',
    count:4
}
if (obj.whatevs == "hi") {
    console.log('ok');
}

for (var i = 0; i < obj.count; i++) {
    console.log(i);
}

You can test to see if a property exists on an object:

const obj = {
    something:'wuttt'
}

if (obj.something) {
    console.log('ok');
}
if (obj.anotherthing){
    console.log('ok');
} else {
    console.log('no go');
}

This is because accessing a property that doesn't exist on an object gives you undefined which is treated as false.

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