How to Write JavaScript Variables and Statements

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So far in this course, we have learned:

  • How to select a web browser for JavaScript development
  • How to install Node.js and npm, the Node package manager
  • How to select a text editor (I use VS Code)
  • How to run JavaScript both in your browser or from an HTML or JavaScript file
  • Some general guidelines on JavaScript syntax

These are all important skills, but we still have not written our first line of JavaScript code!

That changes with this lesson. In this tutorial, you’ll learn how to write JavaScript variables and statements, which are the building blocks of software engineering with JavaScript.

What Are JavaScript Variables?

In computer programming, variables are names that store a specified value.

There are three keywords that can be used to create JavaScript variables:

  • var
  • let
  • const

We’ll explore each keyword in detail in this tutorial. First, let’s talk about some restrictions on variable names.

The JavaScript var Keyword

The var keyword is one way of creating variables in JavaScript. For example, you can create a variable named first with a value of Nick with the following command:


var first = 'Nick';

Once this variable has been created, you can print its value to the console of your web browser with the following JavaScript statement:


console.log(first)

The output of this statement will be:


Nick

The JavaScript let Keyword

The let keyword is another way to declare a variable in JavaScript. As an example, you could create a variable named age with a value of 24 with the following JavaScript statement:


let age = 24;

Once again, we can print the value held within age to the console with the console.log statement like this:


console.log(age);

Here is the output of this command:


24

The JavaScript const Keyword

The JavaScript const keyword is the third (and last) way to declare variables in JavaScript.

Let’s create a variable called last with a value of McCullum using the const keyword:


const last = 'McCullum';

As before, we can print the value of last to the console with the console.log statement like this:


console.log(last);

Here is the output of this statement:


McCullum

Variables declared with the var, let, and const keywords have different characteristics. We will discuss these attributes in the next section of this lesson.

The Differences Between the var, let, and const Keywords in JavaScript

The var, let, and const keywords each behave differently, and understanding the differences in their behavior is an important step in becoming a mature JavaScript developer.

To start, the var and let keywords can be updated. The const keyword cannot.

Let’s consider an example:


//initial variable declaration and assignment

var first = "Nick"

let last = "McCullum"

const age = 24

//update the "first" and "last" variables

first = "Warren"

last = "Buffett"

//log the "first" and "last" variables to the console

console.log(first)

console.log(last)

Notice that the var and let keywords only need to be used the first time a variable is created. If you attempt to modify the variables at a later date using those keywords, your JavaScript code will return an error. Said differently, you do not need to use the var or let keywords when updating a variable’s value.

What do you think happens when we attempt to run this code?

If you guessed that the new values of first and last will be printed to the console, then you are correct! Specifically, here’s what the output would be:


Warren

Buffett

What happens if you try and update the variable that was created with the const keyword? JavaScript will return an error.

As an example, let’s try and update the age variable:


age = 89;

This code returns the following error:


Uncaught TypeError: Assignment to constant variable.

    at <anonymous>:1:5

The second important difference between var, let, and const is a concept called scoping. Scoping refers to where a variable can be accessed within a JavaScript application.

We will have an entire section of this course that dives into JavaScript scoping in greater detail. For now, it is enough to remember that var variables are scoped differently than let and const variables. var variables are function scoped while let and const are block scoped.

One last note on the var, let, and const variables - specifically regarding their history. var variables have been around since JavaScript’s invention in 1995, while let and const variables were introduced in ES6, which was a new version of JavaScript that rolled out in 2015.

If this is a lot to digest, do not worry! We’ll learn more about this later. We will learn much more about variable scope later in this course.

JavaScript Variables and Strict Mode

If you play around with variables enough in JavaScript, you may notice that it is indeed possible to declare a variable without the var, let, or const keywords. As an example, the following code runs successfully:


variable = 'This is my variable!';

You can then print this variable’s value with a console.log statement like this:


console.log(variable);

Which returns:


This is my variable!

What is going on here?

Well, in JavaScript’s early days, the var, let, and const keywords were not as formalized as they are today, so if you’re missing these keywords then the browser will automatically make the variable of type var.

If you start a segment of JavaScript code with 'use strict';, it disables this functionality. As an example, the following code returns an error:


'use strict';

variable = 'This is my variable!';

Here is the specific error that is returned by this code:


VM247:3 Uncaught ReferenceError: variable is not defined

    at <anonymous>:3:10

Strict mode is useful in JavaScript development. Importantly, it is enforced by default when you use JavaScript modules (we’ll learn more about that later). You will probably be writing most of your code using JavaScript modules, so you will not need to include 'use strict'; at the start of each of your JavaScript programs.

When to Use the var, let, and const Keywords in JavaScript

One concept that beginner JavaScript developers sometimes struggle with is the choice of when to use the var, let, and const keywords.

The truth is, there is no hard and fast rule. I do recognize that having some guidelines is helpful nonetheless, so here is what I do:

  • When declaring a new variable, I use the const variable by default
  • If it turns out that I need to change the value of that variable later in my JavaScript program, then I will change the variable type to let
  • There are a few special situations where I may use var, but these are few and far between

I reiterate that these are not rules, but simply my opinion.

JavaScript Variable Naming Conventions

To wrap up this lesson, let’s learn what the best practices are for naming JavaScript variables.

To start, JavaScript variables should generally not contain any special characters like !, ?, or ". There are two exceptions to this. Both $ and _ are permitted to be use in JavaScript variable names.

The other aspect of JavaScript variable naming that you should keep in mind is the concept of camelCase. camelCase is a naming convention in which you start your first word with a lowercase letter, use no spaces, and uppercase the first letter of every word from the second word onwards.

Here are a few examples of camelCase variable names to help you understand this better:


myFullName = 'Nick McCullum';

theNameOfThisCourse = 'JavaScript Fundamentals';

thisWebsiteURL = 'www.nickmccullum.com';

While there are other naming conventions (like snake_case, which is more common in Python development), you will find that the majority of JavaScript developers use camelCase when naming their variables in their applications.

Undefined Variables in JavaScript

You might be wondering what happens if we create variables but do not assign any value to them. For example, what happens when we attempt to run the following code:

let variable;
console.log(variable);

JavaScript has a built-in data type called undefined to help handle this. As you’d expect, that’s exactly what the output of tha last code block is:

undefined

The semantics here are a bit hard to understand, but a variable with value undefined is different than a variable that has not yet been created.

To see this concept in action, try and print a variable in the JavaScript console that you haven’t created yet. You will get the following error:

VM371:1 Uncaught ReferenceError: notAVariable is not defined
    at <anonymous>:1:13

JavaScript also has a second data value to explicitly deal with a variable that is meant to explicitly state that it has no value. This value is null.

Unlike the undefined value, a value of null must be explicitly assigned to a JavaScript variable like this:

const nullVariable = null;

You might be wondering why JavaScript has two different keywords to deal with undefined or null values. This is because the null keyword can be explicitly used to change the value of a keyword that already exists, like this:

let name = 'Nicholas';
name = null;
console.log(name);
//Prints null

Final Thoughts

In this lesson, you learned how to write JavaScript variables and statements. Over the next few lessons, we’ll be diving into each of the major data types in the JavaScript programming language.