Data Types in JS

Learning Goals

  • Review and discuss JavaScript fundamentals
  • Practice working with functions, primitive, and complex data types


  • Data Type A type of data, e.g., String, Object, Boolean, etc.
  • Primitives / Simple Data Types Basic data types like String, Boolean, Number
  • Complex Data Types Data types that are not Primitives. E.g., Objects, Arrays
  • Object A data structure made up of Keys and Values
  • Key The name used to refer to a Value in an Object
  • Value The value referenced by a Key in an Object
  • Array A list of values
  • Function A grouping of executable code. Can be manipulated just like any other Object
  • Method A function that’s defined as a property of an object
  • Scope Where variables are accessible
  • Parameters The variables a function says it will take in when it runs. Declared inside parens
  • Arguments The actual variables a function uses when it runs

What is a Data Type?

In this lesson we’ll talk a bit further about data types in JavaScript. Every language has a concept of included data types, but there are often some differences in how they are handled, what they are called, etc. This can cause some confusion when trying to learn a new language, so we want to make sure we understand them deeply in JavaScript!

Shout ‘em Out!

What data types are you familiar with in JavaScript?

In Your Notebook

How do we know what data type we’re working with?

Dynamically Typed vs. Statically Typed

Programming languages can fall under two categories, either dynamically typed or statically typed.

In a dynamically typed language, the interpreter will infer what kind of data type we’re working with rather than us having to explicity define that. This also means that the data type of a given value can change on you! (which can be a common source of bugs)

In contrast, statically typed languages expect you to explicity state what data type you would like a particular value to be.

How might you describe these terms in your own words? Would JavaScript be considered dynamically typed or statically typed?

If you discovered that JavaScript is a dynamically typed language, you would be correct! Declaring variables in JavaScript might look similar to this:

let count = 5;
let fact = true;

In contrast, a statically typed language like Java would declare these same variables as:

int count = 5
boolean fact = true

Where int says “This variable must be an integer”, count is the name of our variable, and 5 is it’s value. Now count will always represent an integer and we will not be able to change it to a string or other data type later. Same scenario for the boolean on the line below.

How does JavaScript know what data type we’re working with? It infers it based on the way we wrote the value. If JavaScript were a person, it might look at this line of code and ask:

  • is the value wrapped in quotes? -> no -> it is not a string
  • is the value wrapped in hard brackets []? -> no -> it is not an array
  • is the value wrapped in curly brackets {} -> no -> it is not an object
  • is the value undefined, null, true, or false -> no -> it is not undefined, null or a boolean
  • is the value a valid number -> yes -> it must be a number

Type Coercion

In addition to having our data types inferred, dynamically typed languages also allow for the data type of a value to change over time. Consider the following code:

Your Turn

Considering the following, what value and data type will totalCount be? (Write down what you think it will be before trying it out)

let counter = 5;
let newCounter = '10';

let totalCount = counter + newCounter;

Let’s try another scenario. Although very similar, we will now use subtraction. What would the value and data type be for totalCount now?

let counter = 5;
let newCounter = '10';

let totalCount = newCounter - counter;

What we see happen with this code is called type coercion. JavaScript says, “Hmm, you’re trying to add a number and a string together here - I’m going to try my best and make an assumption about what you want here!”. It then coerces the number into a string so that it can concatenate the two strings together more easily. In statically typed languages, we would get an error for trying to do this! But JavaScript is our dear friend that’s trying it’s best.

Simple / Primitive Data Types


Primitive Data Types

  • What are some characteristics of primitive data types?

Working with Primitives

Primitive data types are not objects, have no methods, and are immutable. Let’s uncover a bit more about these properties of a primitive data type. Consider the following code:

let cow = 'moo';
console.log(cow); // what will be logged here?

Turn and talk

  • On line 2, what kind of syntax are we using? Where have we seen this syntax before?
  • What data type is toUpperCase?
  • What will get logged on line 3?

This particular code example seems to fly in the face of what we just learned about primitives. We are using dot notation to call a method on our cow variable, then uppercasing it with that method. You’ll notice, however, that when we log our cow on line 3, we still get the lower cased original version of that string. In order to capture that new format, we would need to reassign the cow variable to that new representation of the string, like so:

An important distinction about immutability is that variables themselves (e.g. cow) are not immutable, but their primitive values are. We can completely reassign cow to a new value in order to store the uppercased version.

Complex Data Types / Objects


Objects are a complex data type provided by JavaScript. It is essentially a map between key value pairs. Keys must be strings and values can be any data type. You can think of an object as a collection of related variables.

Creating Objects

Although there are several ways to create objects, most people prefer to use the object literal notation. (i.e. {}) This is a short and fast way to create a new object.

If we want to access our object later, we need to store it in a variable.

const dog = {};


What are objects made up of?

If we had a lot of information we wanted to store about our dog we might be tempted to create a lot of variables.

let dogName = 'Boris';
let dogAge = 3;
let dogBreed = 'Labradoodle';

How might we store this information in an object literal? Create a variable that uses object literal notation to store the same data.

Adding properties to an object

We can add properties to an object when it is created or we can add properties after an object is created. We can only create a property on an object if we give it a value.

const dog = {
  // property name  | property value
	name: 'Boris'

// OR

const dog = {}; = 'Boris';

Dot notation vs Bracket Notation

Dot notation is great because the syntax is short and it is easy to read. It should be used whenever you know the exact name of the property you are accessing:

const dog = {}; = 'Boris';

Bracket notation is great if you don’t know what the key is or if the key might change:

// Example 1:
const dog = {};
let property = 'name';

dog[property] = 'Boris';

console.log(dog); // { name: 'Boris' };

property = 'breed';

dog[property] = 'Labradoodle';

console.log(dog); // { name: 'Boris', breed: 'Labradoodle' };
// Example 2:
const storeItems = [ 'hammer', 'chisel', 'rake' ];
const store = {
	hammer: 5,
	chisel: 3,
	rake: 7

let totalCost = 0;

for (let i = 0; i < storeItems.length; i++) {
	let itemName = storeItems[i];
	let itemPrice = store[ itemName ];  // using bracket notation because property is changing

	totalCost += itemPrice;



Bracket notation gives us fewer limitations than dot notation. Using bracket notation allows us to set and access keys that have spaces or other characters. Typically you want to avoid doing this but you might on occasion find you have to do it:

const dog = {};
dog['first name'] = 'Antigone';

console.log(dog); // {first name: 'Antigone'}

Your Turn (in pairs)

With a partner, complete the following exercise:

Given the following object and variable, access the value 'Apis mellifera' in the three different ways we discussed (dot notation, bracket string literal, bracket string variable):

const honeybee = {
  scientificName: 'Apis mellifera'

const fancyName = 'scientificName';


Arrays are list-like objects, used for storing a list of related values. If you were making a list of pizza toppings, you might store them in an array like so:

const toppings = ['cheese', 'peppers', 'onions', 'garlic'];

Remember we said arrays are actually objects under the hood. Though it might not seem like arrays have a set of key-value pairs like the objects we just discussed, each value in the array actually has a numbered key associated with it that counts from 0. For example, written as an object, the toppings array might look like this:

const toppings = {
	0: 'cheese',
	1: 'peppers',
	2: 'onions',
	3: 'garlic'

This syntax is a lot more verbose, and the numbered keys don’t really give us any useful information except where the value exists within the array. This can be important when we want to use some of the built-in helper methods that come with arrays.


Functions are objects that contain JS code which can be repeatedly invoked/executed.

A function declaration is declared using the keyword function and the following syntax:

function sayHi () {
	let greeting = 'Hi!';

Write the above function as an ES6 arrow function! Only look below once you have completed!

// ES6 "fat arrow" style function
const sayHi = () => {
	let greeting = 'Hi!';


Function Scope

function sayHi () {
	let greeting = 'Hi!';

	console.log(greeting); // 'Hi!'

console.log(greeting); // Reference Error: greeting is not defined

Any variables which are created in the function only exist in that function. You do not have access to them outside of the function in which they were declared.

Parameters / Arguments

Consider the following

function sayHi (name) {
	let greeting = 'Hi, ' + name + '!';



Is name a parameter or argument? What about 'Bob'?

Returning something out of a function

If we want to create something in the function and use it outside of our function’s scope, we need to return it out of the function.

function makeGreeting (name) {
	let greeting = 'Hi, ' + name + '!';

	return greeting;

let mabelGreeting = makeGreeting('Mabel');



If we are returning a value out of a function that we wish to use elsewhere in our code, it must be stored in a way that will allow us to reference it later. The easiest way to do this is to capture the value with a variable. In the above example, we are creating a new variable named mabelGreeting in which we are storing our newly created greeting (aka the value being returned by invoking makeGreeting with an argument of 'Mabel').

Checks for Understanding

  • What are some characteristics of primitive data types?
  • What are the complex data types in JS?
  • When would you store information in an object vs an array? Why?

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