JS I - Data Types, Variables, Conditionals, and Functions

Learning Goals

In this lesson we’ll cover:

  • JavaScript primitive data types
  • What a variable is and how to use it
  • Adding variables to strings (concatenation and interpolation)
  • Operators and how to use them
  • Conditionals and when to use them
  • How to declare and call functions with or without parameters


  • Data Type A kind of data, defined by the values it can hold and the operations that can be done on it
  • Primitive type A kind of data type. Primitives in Javascript are [string, number, boolean, null, undefined, symbol]
  • Variable A container for a value. The main building block for all programming
  • Declare Creating a new variable (distinct from assignment)
  • Assignment Assigning a value to a variable
  • Type coercion The process of converting a value from one type to another (such as a number to a string).
  • Concatenation The binding of multiple strings together using the + string operator
  • Interpolation - The process of injecting a variable directly into a string.
  • Template literal - Template literals are string literals that provide an easy way to interpolate a variable or expression into a string.
  • Statement An executable piece of code
  • Expression An executable piece of code that resolves to a single value, the result of which may be assigned to a variable.
  • Operator Symbols that are used to assign, compare, and perform operations
  • Conditional An expression that evaluates to true or false, or a control flow statement that executes code
  • Function A predefined and reusable group of behavior
  • Call/Invoke Running a function
  • Parameters The variables declared in a function definition
  • Arguments The variables passed to a function when it’s called/invoked

History of JavaScript

Way back in the early days of the web, Brendan Eich created JavaScript. Legend has it that he wrote it in 10 days in 1995 while was was working as an engineer at Netscape. The language was first released with Netscape 2 in 1996. The name ‘JavaScript’ was a oh-so-clever marketing push to try to pick up on the momentum of Sun Microsystem’s popular language Java, but instead it lead to years of confusion about the names of these two mostly unrelated programming languages. If you’d like to learn more about where JS came from, take a look here.

What is JavaScript and Why?

JavaScript was created to make the web more dynamic. It is an object-oriented scripting language made to run inside a host environment like a web browser and provide programatic control over the objects of that environment. For example, when you click a button and you want something about the webpage to change, you will use JavaScript.

JavaScript can be client-side and server-side, meaning that it can be used to control user-facing interfaces as well as handle the server-side extensions that connect with a database.

It’s a highly versatile and flexible language, and has become the most commonly used language of the web.

Now that we know a little bit about JavaScript, let’s talk about how to actually use it!

Side Note: The console

Browsers give developers a set of handy tools, aptly named Developer Tools. One such tool is the console, an environment where you can run Javascript inside the browser. The console is one of the most useful ways to interact with your code from the browser, and we’ll be using it to demonstrate and practice concepts today.

To open the console in Chrome, either:

  • Click View > Developer > Javascript Console
  • Or use the hotkeys command+option+J / command+option+I

Data Types

There are different data types in JavaScript. It’s important to understand what type of data you’re dealing with as you’re writing code, and knowing the types of data are available to you is the first step.

Javascript has six primitive data types:

  • null
  • undefined
  • Boolean
  • Number
  • String
  • Symbol (new in ECMAScript 6 we do not need to worry about this one)

Let’s go over what each of these is.

Null Data Type

  • The datatype null only has one value which is null.
  • Shows an intentional absence of a value
  • Often is used when creating a variable that could have a value later.

Undefined Data Type

  • The datatype undefined only has one value which is undefined.
  • A variable which has not been assigned a value has a datatype of undefined.
  • A function returns undefined unless another value is returned.

Boolean Data Type

  • A boolean has one of two values: true or false.
    • Can act like a light switch: it’s either on or off.
  • Booleans can be very useful to help determine which part of a script should be run (as in conditionals, which we will talk about later).

Numeric Data Type

  • The numeric data type deals with all numbers.
    • No need for commas EX: three thousand four hundred and seventy five would be 3475
  • Negatives and decimals (also called “floats”) are written like -3475 and 0.5
    • Unlike other programming languages, there is no distinction between integer and floating-point values
  • Important Note: do not enclose a number data type in quotes.

String Data Type

  • The string data type handles letters and other characters.
    • Always enclosed in quotes EX: "I am a string!"
  • Can use either single or double quotes to wrap a string
    • Make sure opening and closing quotes match
    • Be consistent throughout a project

Strings can be used for any kind of text. We use them regularly to add text to a page.

Your Turn

Take a couple of minutes to pair up and explain these the following data types to your neighbor: null, undefined, boolean, number, and string. Take turns, and make sure both of you have a chance to explain, practicing correct vocabulary and talking about code. Also practice using your console to show the different data types.

Variables in JavaScript

As we’re writing code, there are many times when we want to be able to store a value so that we have the ability to reuse it in multiple places. Writing the same thing over and over is a pain, and it’s common that we need to have access to the same information more than once. A variable lets us do just that!

What is a variable?

A variable is a place to store values. When we write scripts (a set of instructions for a computer to follow), we need to temporarily store small pieces of data. We store that data in variables. “Variable” is a good name for this concept because it indicates the stored data can change (or vary) each time a script is run.

A variable is, at its most simple, a declaration. It’s made up of two parts: the variable keyword, var (a special word that the JavaScript interpreter knows is used to create a variable), and the variable name, which can be whatever you choose. Let’s see what that looks like in the console:

var myVariableName;

We see that we have the variable keyword, and we’ve set the variable name to myVariableName.

When we write a very simple variable like this, it’s called declaring the variable.

Rules for Naming Variables

  • Names must begin with a letter, dollar sign, or an underscore. They cannot begin with a number.
  • Names can contain any of the above characters plus a number, but you cannot use a dash (-) or a period (.) within the name.
  • You cannot use keywords or reserved words (such as var or for).
  • All variables are case sensitive.
  • Use names that describe the kind of information you plan to assign the variable.
  • If your variable is made up of more than one word, then use camel case for every word AFTER the first word, which should be lower case ie. thisIsMyVariableName.

Your Turn (in the console)

Declare 2 variables, one named “quantity” and one named “mythicalCreature”

Variable Values: Assignment

Notice when we first create a variable, its value is undefined because it doesn’t have a value yet. This is because we have to set a value! Let’s add a value to a excellentHuman variable:

var excellentHuman = "Fred Rogers";

Notice after our var keyword and our variable name, we have an equals sign, =. This is called the assignment operator, because we use it to assign a value to our variable declaration. Without the assignment operator and the variable value, the variable is considered undefined.

Let’s test these additions in our console!

  • Step 1: declare a variable and it’s value
  • Step 2: call the variable
  • Step 3: dance for joy at seeing our assigned value of “Fred Rogers”

Your Turn (in the console)

  • Assign values to your "quantity" and "mythicalCreature" variables
  • WWhat data type did you assign as the value in your "quantity" and "mythicalCreature" variables?
  • Assign values to your "quantity" and "mythicalCreature" variables.
  • Create three more variables, and give them each different data types (numbers, strings, booleans)

Using Variables Together

Now that we know about different data types and have values assigned to both of our variables, let’s dive into using them together!

var quantity = 3;
var mythicalCreature = " unicorns";
var creatureCount = quantity + mythicalCreature;

We have a variable called “creatureCount” and assign the value as our quantity and our mythicalCreature. Looking at creatureCount, we’ll notice the value is "3 unicorns". The reason this works even though the first value is a number and the second is a string, is a result of type coercion. Javascript is essentially trying to help us by converting the number into a string so that they can combine the two together.

Your Turn (in the console)

Make 4 new variables. Assign values with numeric data types to two of them, and string data types to the others. See what happens when you combine the two numeric variables together vs what happens when you combine the two string variables.

Adding Variable Values to Strings

There are two different ways to add values to strings.

1. Via Concatenation

In the example above, we used a + as a string operator to combine the values of two different variables. This is called concatenation, which is a series of values linked together.

Here’s another example

var food = "burrito";
var sentence = "I had a " + food + " for breakfast."

In this instance, we’re concatenating multiple values including two strings and a variable in order for the sentence to come out as "I had a burrito for breakfast." This can make our string more dynamic depending on what value is in the variable.

2. Via Interpolation

Another way of adding values to strings is through template literals. Template literals are special strings that allow us to interpolate information. They use backticks instead of normal quotes and use a combination of a dollar sign followed by curly braces. ie. ${quantity} ${mythicalCreature}

We can concatenate or interpolate html tags, text, numbers, and variable values. Let’s revisit our example above to create a more readable phrase as the value of our creatureCount variable:

var quantity = 3;
var mythicalCreature = "unicorns";

// using concatenation
var creatureCount = "<p>I have " + quantity + " very fancy " + mythicalCreature + "</p>"

// using a template literal
var creatureCount = `<p>I have ${quantity} very fancy ${mythicalCreature}</p>`

This is very useful when we want to append data to our webpage, because it means we can write out an entire HTML tag and add the data from our variable.

One important thing to keep in mind is that template literals are not as widely supported as concatenation.


A script is a series of instructions that a computer can follow one by one. Each individual instruction is known as a statement. Each statement in JavaScript is followed by a semicolon.

Example statements:



An expression results in a single value. Expressions can use operators to create this single value. Here are some example expressions:

This expression results in “apple”


This expression results in 5

2 + 3

What is the difference?

In simpler terms, a statement performs some kind of action. It does not return anything. An expression produces a value and can be written wherever a value is expected.

Your Turn (in your notebook)

Write down in your own words what both a statement and expression are. What is the difference between the two?


Expressions rely on operators to calculate their single value. There are 5 basic types of operators to get you started:

  1. Assignment operators assign a value to a variable. (hint: you’ve got these down already) var color = 'magenta';
  2. Arithmetic operators perform basic math. var addTwo = 2 + 2;
  3. String operators combine strings. var greeting = 'Hello! ' + 'Nice to meet you.';
  4. Comparison operators compare two values and return a true or false. var buy = 3 > 5; // Value of buy is false
  5. Logical operators combines expressions and return a Boolean value of true or false. var buy = (5 > 3) && (2 < 4);

Your Turn

Practice writing out atleast one of each using arithmetic, string, comparison, and logical operators with the person next to you. Explain what each one is doing and how it is working.


Sometimes we want to perform an action based on some kind of condition. In English, we can say “If this thing is true, then do that.” In JavaScript, conditionals are written very similary and allow us to take a certain path in our program.

To use conditionals, we first need to use expressions, operators, and statements. Let’s head on over to repl and practice writing them out there!

Basic Conditional Structure

Let’s now look at the basic structure of a conditional:

if (expression) {
} else {

If the expression evaluates to true, then the statement(s) for that condition will run. Otherwise, if the expression is false, then the statement(s) will not run at all. The expression will usually contain an operator to make a comparison (that evaluates to true or false).

Some examples of expressions we could use for a conditional are:

  • myNum < 5
  • userCity === "Denver"
  • isTired === true

Now for some real conditional examples.

var hoursOfSleep = 8;

if (hoursOfSleep < 6) {
  console.log('I am groggy.');
} else {
  console.log('I feel fantastic!');
var nextLocation = "home";

if (nextLocation === "home") {
  console.log("It's been a long day, let's go home!");
} else if (nextLocation === "work") {
  console.log("Good morning, finding the fastest route to work!");
} else {
  console.log("Finding location.  Found it!  Let's go!");

Your Turn (in the console)

Write a conditional that logs a different message based on your favorite kind of animal.


Functions are a way to group statements together to perform a specific task. Functions are reusable blocks of code. To create a function, you must give it a name and then write the statements required for the function to achieve its task inside the function’s curly braces. Let’s work through the pieces and parts of a function.

Anatomy of a Function:

function makeStatements() {
  var firstStatement = 'Pizza is AMAZING!';
  console.log('Burritos taste delicious!');
  • You declare a function using the keyword function.
  • Since functions perform an action, it is generally encouraged to use an action verb in the function name. i.e. getSize, saveNote, generateRandomNumber.
  • You can name the function anything you want, but it must include a set of parentheses after the name, which can be empty or accept parameters. (we’ll go over parameters shortly, but for now let’s keep it empty)
  • You must follow the parentheses with a set of curly braces, which act as bookends to hold the set of statements you want the function to run when it is called.
  • In order to call a function, type the function name with it’s parentheses and any associated arguments (Take a look below)

Invoke a Function:

// Invoke a function

When this code is read, makeStatements() is “invoked”, all three statements within the function’s body (those curly braces) get run, one line at a time. What do you think will happen if we swap the 2nd and 3rd statements?

Your Turn

With the person across from you, create your own functions in the console:

  • Write a function that logs to the console a message of "YO!", and then logs a sum of eight different integers.
  • Write a function that assigns three different math equations to three different variables, then logs the sum of the values of all three variables.
  • Write a function that declares a `firstName` variable and a `lastName` variable, then logs a message that incorporates the full name, and then logs a random number to the console.

Passing Information to a Function:

Sometimes you need to give a function some information in order for it to do its job. You can give that function the information it needs by providing parameters in the function declaration. These are place holders that you identify with appropriately named labels inside the parentheses of your named function. The words you use for your parameters act like variables INSIDE the function, which means they serve as a means to pass values.

Parameters vs. Arguments

Its a subtle difference. Basically, when you declare a function, and you stipulate the function will accept some bits of information, those are parameters. Then, when you pass the values of the parameters, those are called arguments. Like this:

// parameters named on declaration of function
function createDreamCar(make, model) {
  return "My dream car is a " + make + " " + model;

// arguments "Audi" and "R8" passed into a called function
createDreamCar("Audi", "R8");

Getting A Value from Functions:

Some functions return information to the code that called them. Wait - what? When a function performs a calculation, like 2 + 2, it will return the “answer” of 4, right?I still need the keyword return in order to return that value back from a function. Let’s work with some return statements in functions with parameters in the console:

function addTwoNumbers(num1, num2) {
  num1 + num2;

function addTwoNumbers(num1, num2) {
  return num1 + num2;

function addTwoNumbers(num1, num2) {
  return num1 + num2;

function addTwoNumbers(num1, num2) {
  return num1 + num2;

Notice in the last example that when the return statement is called in a function, it ends function execution (the following lines do not run), and it specifies a value to be returned to the function caller. If the expression is omitted, undefined is returned instead. Here is the basic structure:

// This is the formula:
return [expression];
// The expression to return. If the return is omitted, undefined is returned instead. The [] are meant to show that the expression is dynamic.

The following return statements are all examples that would break function execution:

return true;
return false;
return x;
return x + y / 3;

You Do

  • Create a function called `saySomething` that requires no parameters. This function should return a message of your choice.
  • Create a function called `showFavorites` that requires three parameters - a food, a movie, and an animal. Your function should return a string that uses either concatenation or interpolation to return the three favorite things in a string.
  • Create a function called `calculateAge` that takes a single parameter - a year. Your function should return the difference in years from the birth year from our current year, 2019.


  1. What are the six primitive data types?
  2. How are variables useful and what is an example of one that has a value assigned to it?
  3. Write out an example of string concatenation. Now write that same example using a template literal.
  4. Write out the basic structure of an if/else conditional.
  5. Write out a function declaration and then what it looks like to invoke it.
  6. What is the difference between parameters and arguments?

Additional Resources & Practice

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