In this lesson, we'll introduce JavaScript variables and explain how to declare, assign, and access them.
- Define a variable
- Name variables in JavaScript
- Initialize variables in JavaScript
- Identify when to use
const
,let
, andvar
for declaring variables
A variable is a container in which we can store values for later retrieval.
Imagine a box that can hold any type of data: a number, string, true/false,
object — even an undefined
. We take some data that we want to store, place it
inside the box, and hand the box off to JavaScript, which stores it in memory.
All done! Our data is safely stored until we need to access it again.
But wait! When we ask for the data back, how will JavaScript know which box to retrieve? We need to assign a name to our variable — a label for our box — so that we can tell the engine exactly which piece of stored data we want.
Variable names in JavaScript can sometimes be complicated, but if you follow these three rules you'll be fine:
- Start every variable name with a lowercase letter. Variable names starting with a number are not valid.
- Don't use spaces —
camelCaseYourVariableNames
(see the camel humps?) instead ofsnake_casing_them
(like the underscore is a snake that swallowed the words). - Don't use JavaScript reserved words or future reserved words.
It's important to note that case matters, so javaScript
, javascript
,
JavaScript
, and JAVASCRIPT
are four different variables.
The word var
is a special word in JavaScript. It means that the word that
comes next is a variable name, an identifier for a box that we will put data
in. It was the first way to declare a variable. For most JavaScript written
before 2015, only var
is used for variables.
Now, let's use var
.
var pi;
//=> undefined
...and JavaScript sets aside a chunk of memory to store the declared variable. Then, we assign a value to that variable:
pi = 3.14159;
//=> 3.14159
We can package both of the initialization steps — declaration and assignment — in a single line of code:
var pi = 3.14159; //=> undefined
Both of these methods are commonly used, so you could see and/or use either one.
To retrieve a declared variable, simply refer to its name:
pi;
//=> 3.14159
Upon declaration, all variables are automatically assigned the value of
undefined
. It's only after we assign a new value that the variable will
contain something other than undefined. We can use typeof
to check the data
type of the value currently stored in a variable:
var language;
//=> undefined
typeof language;
//=> "undefined"
language = "JavaScript";
//=> "JavaScript"
typeof language;
//=> "string"
Top Tip: When writing JavaScript code, it's good practice to never set a
variable equal to undefined
. Variables will be undefined
until we explicitly
assign a value, so encountering an undefined
variable is a strong signal that
the variable was declared before being used (or, as programmers say, being "referenced"). That's valuable information that we can use while debugging, and it comes at no
additional cost to us.
Once a variable has been created with var
, we can reassign it to our heart's
content:
var pi = 3.14159;
//=> undefined
typeof pi;
//=> "number"
pi = "the ratio between a circle's circumference and diameter";
//=> "the ratio between a circle's circumference and diameter"
typeof pi;
//=> "string"
The data that's stored in our variable might change over time, but at any moment we can retrieve its current contents:
var language = "Mocha";
//=> undefined
language = "LiveScript";
//=> "LiveScript";
language = "JavaScript";
//=> "JavaScript";
language;
//=> "JavaScript";
Because of its ubiquity in legacy code and StackOverflow posts, it's important
to get to know var
. However, as we alluded to earlier, there is almost no
reason to use var
with the features JavaScript has post-2015. var
comes with
a ton of baggage in the form of scope issues (which we will discuss in the
lesson on scope in JavaScript) and allows developers to play a little too
fast and loose with variable declarations.
For example, with var
, no error is thrown if you declare a variable twice:
var language = "Ruby";
//=> undefined
var language = "JavaScript";
//=> undefined
language;
//=> "JavaScript"
This is bad! There's no reason to declare a variable twice, and it's usually a mistake by a developer unaware that the variable had already been declared.
ES2015 introduced two new ways to create variables: let
and const
. Both
solve all of var
's scope issues, which, again, we'll cover in the lesson on
scope in JavaScript. Both also throw an error if you try to declare the same
variable a second time:
let pi = 3.14159;
//=> undefined
let pi = "the ratio between a circle's circumference and diameter";
//=> Uncaught SyntaxError: Identifier 'pi' has already been declared
Just like with var
, we can still reassign a variable declared with let
:
let pi = 3.14159;
//=> undefined
pi = "the ratio between a circle's circumference and diameter";
//=> "the ratio between a circle's circumference and diameter"
typeof pi;
//=> "string"
Using let
instead of var
will help you avoid silly errors like declaring the
same variable at two different places within your code, but there's an even
better option to use as your default: const
.
Declaring a variable with the const
reserved word means that not only can it
not be redeclared but it also cannot be reassigned. This is a good thing
for three reasons:
- When we assign a primitive value (any type of data except an object) to a variable declared with
const
, we know that variable will always contain the same value. - When we assign an object to a variable declared with
const
, we know that variable will always point to the same object (though the object's properties can still be modified — more on this in the lesson about objects in JavaScript). - When another developer looks at our code and sees a
const
declaration, they immediately know that variable points to the same object or has the same value every other time it's referenced in the program. For variables declared withlet
orvar
, the developer cannot be so sure and will have to keep track of how those variables change throughout the program. The extra information provided byconst
is valuable, and it comes at no extra cost to you! Just useconst
as much as possible and reap the benefits.
const pi = 3.14159;
//=> undefined
pi = 2.71828;
//=> Uncaught TypeError: Assignment to constant variable.
NOTE: With let
, it's possible to declare a variable without assigning a value, just like var
:
let pi;
//=> undefined
pi = 3.14159;
//=> 3.14159
However, because const
doesn't allow reassignment after the variable is initialized, we must assign a value right away:
const pi;
//=> Uncaught SyntaxError: Missing initializer in const declaration
const pi = 3.14159;
//=> undefined
As your JavaScript powers increase with experience, you'll develop a more nuanced understanding of what to use where. However, for now, this is a good rule of thumb:
- Use
var
... never. - Use
let
... when you know the value of a variable will change. For example, acounter
variable that starts at0
and is subsequently incremented to1
,2
,3
, and so on. In the lessons on looping and iteration in JavaScript,let
will have its moment in the spotlight. - Use
const
... for every other variable.
Best practice is to always declare variables with const
and then, if you later
realize that the value has to change over the course of your program, circle
back to change it to let
.
NOTE ON CHROME 80: With the update to Chrome 80, let
is now able to be redeclared in the Chrome console (and only your console). Keep this in mind when you're testing out your code! For more on this, check out this article.
- MDN — Language basics crash course: Variables
- Valid JavaScript variable names in ECMAScript 6
- MDN —
var
- MDN —
let
- MDN —
const
- JavaScript ES6+:
var
,let
, orconst
? - Google Chrome 80 Updates
We covered what a variable is, how to initialize and retrieve it, and how to assign its values. We also looked at best practices for using var
, let
and const
.