Variables are one of the major features of computer programming and we will use them all the time. A variable is just a container that stores some information such as a number, an object, or a string.
var is short for variable. This is the traditional way of declaring a variable but it has some longstanding issues so we won't use this option.
let is a more modern way to declare a variable whose value will change.
const is a more modern way to declare a variable whose value will never change.
We will learn more about the advantages of
const in this lesson, while we will learn about the major disadvantage of
var in a future lesson.
In the example above, we declare a variable called
myNumber. It doesn't have a value yet. In fact, if we check its value in the console, it will be
undefined. Also, note the semicolon at the end of the line. For now, we will always be including semicolons at the end of each line. We will let you know when that changes.
Note that the first word
my is lowercased while the second word
Number is capitalized. This naming convention is called lower camel case because the lower and upper case letters look a little like the humps on a camel. (There are other camel case naming conventions such as upper camel case, too, which will learn about when they are needed.)
With lower camel case, we always lowercase the first letter of the first word but then capitalize the first letter of any other word in the variable.
Here's another example of lower camel case:
In general, we want to keep variable names short and concise, which the example above certainly doesn't do. However, it does illustrate how lower camel case looks.
In the example above, we declared a variable without assigning it a value. We could've declared the variable and assigned it a value at the same time like this:
var myNumber = 45; > undefined
In the example above,
myNumber is a number, but as we mentioned before, a variable can hold many different kinds of objects. Of course we'd want to name it differently if it's something other than a number - otherwise our variable names would confuse other developers.
var myNumber = 45; is a statement. A statement doesn't return a value, which means its return value is just
That doesn't mean
undefined, though. Type the variable name in the console:
myNumber; > 45
The console returns the value of
myNumber, which is
We can modify
myNumber if we want:
myNumber = myNumber + 5; > 50
Note that we don't use
var again when we modify
myNumber. We only use
var when we are declaring a variable - that is, the first time it shows up in our code.
Think of a variable as being like an actor that comes on stage and introduces their character. Over the course of the play, their character may change but they don't need to introduce themselves again - like a variable, they've already been declared. We'd be pretty annoyed if they introduced themselves again every time they came out.
Let's take another look at the example above:
myNumber = myNumber + 5;
If we look at this from the perspective of using variables in math class in school, this would be mathematically impossible. But things are a little different in programming and this is one of those things that takes a little getting used to. When we modify a variable, we usually want to take the old value of the variable, make a change to it, and then have the variable be equal to the new value.
This is called reassigning a variable. It just means we are changing the value of the variable to something else. In the example above, we are doing the following:
New Value = Original Value + Change We Want to Make to Original Value
We will get plenty of practice with reassigning variables - though many developers don't think reassigning variables is a great idea. We will cover this more when we get to
Here's a little shortcut we can do that's exactly the same thing as
myNumber = myNumber + 5:
myNumber += 5;
Before we move on, what do you think happens to the value of
myNumber if we do this?
myNumber + 5; > 50
The console will correctly return the new value. However, if we check the value of
myNumber, we will see that it hasn't changed. That's because we didn't actually reassign the value of
myNumber. To actually change the value, you can probably guess by this point what we need to do:
myNumber = myNumber + 5;
Alternatively, we can use the shorthand
Let's look at another thing we can do with variables. We can combine several together to make something new. Here's an example:
var num1 = 1; var num2 = 2; var num3 = num1 + num2;
In the example above, we assign values to two different variables. Finally, we add those two variables together and assign that value to a new variable.
In short, we can do anything with variables that we'd do with the things contained inside those variables. If the variables contain numbers, we can do anything that we can do with numbers. If they contain strings, we can do anything we'd do with strings. And so on and on.
It's important to know about
var because you will see it frequently in code examples and legacy code. It was the only option for declaring a variable up until 2015, and even after that, it's taken additional time for some developers to change their practices and update old code.
However, we won't be using
var at Epicodus - as we mentioned before, it has some problems that
var, and you shouldn't, either!
let is an ES6 feature that came out in 2015. Like
const, it has a couple of major advantages over
var. First, it's more descriptive. We will discuss that further in this lesson. Second, it doesn't have the problems with scope that
var has. We will cover these specific scoping issues later this week when we get to branching.
In most ways, both
const work exactly the same as
var. Let's look at the same example we used with
var, but this time we'll use
let instead. (You'll need to refresh the browser to do this in the console or you'll get an error. We'll discuss this error towards the end of this lesson.)
let myNumber = 45; myNumber = myNumber + 5; > 50
As we mentioned at the beginning of this lesson, we can use
let to signify a variable that will change over time. As you can see,
let already has a huge advantage over
var. Any developer can tell just by looking at it that the value of the variable will change at some point.
A big part of being a good developer is good communication. Part of good communication is code that clearly shows what it's meant to do. If another developer sees
let in front of a variable, they've already learned something important about that variable that they wouldn't have known with just
var. It's going to change over time. Now they might want to see exactly where it's going to change and how - especially if they are hunting down a bug or refactoring code.
Finally, we have
const, which is short for constant. As the name implies, a
const is a variable that doesn't change over time - it stays constant. For this reason, there are a few ways in which
const works differently than
Let's take a look at
const in action (refresh your browser first):
const myNumber = 45; myNumber = myNumber + 5;
If we try to do this, we get an error:
Uncaught TypeError: Assignment to constant variable.
We can't reassign
myNumber because it is a constant. This is a very useful advantage that a
const provides that we can't get with
var. It will "freeze" the value of our variable, which is very nice if we want to make sure it doesn't change.
As you might expect, you can't just declare a constant without assigning a value to it. Try this out in the console:
We'll get the following error:
Uncaught SyntaxError: Missing initializer in const declaration
This just means that when we create a constant, we have to assign a value to it. That's because we aren't ever supposed to reassign new values to constants. Even if a variable is declared without a value, reassigning it to have a value is still changing it.
Now let's get back to our example above. What if we want to compute a new value based on
myNumber? We can do this:
const myNumber = 45; const myNewNumber = myNumber + 5; myNewNumber; > 50
We just need to create a new variable to hold our new value. Meanwhile,
myNumber doesn't change - it is still
45. This is because we didn't reassign
myNumber. Instead, we declared a new
myNewNumber and assigned the value to our new variable.
Many developers believe that
const is the only way to go and that we shouldn't use
var at all. This is because when we reassign variables, we might introduce a problem that's hard to track down. With
var, we can never be sure of the value of a variable unless we check it, since it's always possible it will change. With
const, we assign a value to a variable and then we know what it is. We don't have to worry about it changing!
In the example above, we don't have to worry whether
myNumber is 45, 50, or something else.
myNumber will always be 45,
myNewNumber will always be 50, and we can always make good choices about our variable naming to ensure that other developers understand how the values are changing. It's another opportunity to communicate clearly, which will make everyone happy.
We recommend using
const wherever possible, but you may use both
const to declare variables at this point. For now, there is one hard and fast rule - don't use
let if the variable's value won't change in your application. Even though things will work fine, that's not clear communication. That's like saying, "yeah, this is gonna change... no, wait, never mind."
Before we move on, let's take a look at another advantage of
var. As we discussed earlier in this lesson, we should only declare a variable once.
Try doing the following in the console (one at a time, pressing
Enter after each entry):
var myNumber = 1; var myNumber = 2; let myNumber = 3; const myNumber = 4;
When we declare
myNumber a second time with
Here's another way of putting it. If two Janes work at your company, there's a good chance you'll cause confusion if you always refer to both of them as Jane without any clarification about which Jane you're referring to. Did you mean Jane the senior developer or Jane the data analyst? You might need to refer to them as Jane M. and Jane S. instead.
Returning to our example above, when we use
const to try to declare a variable that's already been declared, we get the following error:
Uncaught SyntaxError: Identifier 'myNumber' has already been declared
This is a good error!
const won't let us make this mistake.
Going back to our example about two Janes, this would be like someone stopping us and asking, "Wait, we are still talking about Jane S., right?" And then, to clarify, we'd say, "No, actually, I'm talking about Jane M. now. She's the one that will be working on this new project."
myNumber = 10;
In the example above, we don't use
However, this is very bad for two reasons.
Reason #1 has to do with a slightly more advanced concept called scope that we will learn later this week. We won't cover that in depth now, but it's enough to say that if we declare a variable in this way, it won't just be where you think it is, it'll be everywhere in your code, potentially causing trouble. Going back to our coworker Jane, it would be a little like the team lead accidentally assigning her to all of the projects at our company instead of just the one she's focusing on.
Reason #2 goes back to clear communication.
const communicate very clearly to other developers that a variable is being declared. Writing
const myNumber in our code is like saying "Hey everyone, I'm introducing a new variable that should never change called myNumber."
However, if we declared
myNumber on its own, another developer might think it's been declared elsewhere. We won't even be communicating that we declared a new variable. That is poor communication and it will make other developers upset.
So going forward, make sure to use
const (and preferably
Variable: Variables can be thought of as containers used to store information. They allow for a way to label data with a descriptive name.
Lower camel case: Use
Set a variable equal to a number:
let myNumber = 45;
Use a variable without modifying its value:
favoriteNumber * 4;
Use a variable and modify it:
favoriteNumber = favoriteNumber * 4;
favoriteNumber *= 4;
Use multiple variables:
const myNumber = 45; const otherNumber = 12; myNumber + otherNumber;
Variables should begin with a letter.
Variables are case sensitive (
myNumber is a different variable than
Use clear names that describe the value being stored like
Always name your variables in a manner that will be easy for other developers to understand. Avoid vague letters or initials. (For example:
const x = 45 doesn't tell us what the value is. Is 45 an age, a distance, size, a time?...)
Lesson 7 of 61
Last updated April 7, 2021