JavaScript Variables
Variables = Data Containers
JavaScript variables are containers for data.
JavaScript variables can be declared in 4 ways:
Modern JavaScript
- Using
let - Using
const
Older JavaScript
- Using
var(Not Recommended) - Automatically (Not Recommended)
From the examples you can guess:
- x contains (or stores) the value 5
- y contains (or stores) the value 6
- z contains (or stores) the value 11
Variables are labels for data values.
Variables are containers for storing data.
JavaScript Identifiers
Variables are identified with unique names called identifiers.
Names can be short like x, y, z.
Names can be descriptive age, sum, carName.
The rules for constructing names (identifiers) are:
- Names can contain letters, digits, underscores, and dollar signs.
- Names must begin with a letter, a $ sign or an underscore (_).
- Names are case sensitive (X is different from x).
- Reserved words (JavaScript keywords) cannot be used as names.
Note
Numbers are not allowed as the first character in names.
This way JavaScript can easily distinguish identifiers from numbers.
JavaScript Underscore (_)
JavaScript treats underscore as a letter.
Identifiers containing _ are valid variable names:
A convention among professional programmers is to start a name with underscore for "private" variables.
JavaScript Dollar Sign $
JavaScript also treats a dollar sign as a letter.
Identifiers containing $ are valid variable names:
Using the $ is not very common in JavaScript, but professional programmers often use it as an alias for the main function in JavaScript libraries.
Declaring JavaScript Variables
Creating a variable in JavaScript is called declaring a variable.
You declare a JavaScript variable with the let keyword or
the const keyword.
Declaring a Variable Using let
let carName;
After the declaration, the variable has no value (technically it is undefined).
To assign a value to the variable, use the equal sign:
carName = "Volvo";
Most often you will assign a value to the variable when you declare it:
Example
Create a variable called carName and assign the value "Volvo" to it:
let carName = "Volvo";
Try it Yourself »
Declaring a Variable Using const
Always use const if the value should not be changed
const carName = "Volvo";
The two variables price1 and price2
are declared with the const keyword.
The values of price1 and price2 cannot be changed.
The variable total is declared with the let keyword.
The value of total can be changed.
Declaring a Variable Automatically
Undeclared variables are automatically declared when first used:
It's a good programming practice to declare all variables at the beginning of a script.
Declaring a Variable Using var
The var keyword was used in all JavaScript code before 2015.
The let and const keywords were new to JavaScript in 2015.
When to Use var, let, or const?
1. Always declare variables
2. Always use const if the value should not be changed
3. Always use const if the type should not be changed (Arrays and Objects)
4. Only use let if you cannot use const
5. Never use var if you can use let or const.
JavaScript Data Types
JavaScript variables can hold 8 datatypes, but for now, just think of numbers and strings.
Strings are text written inside quotes.
Numbers are written without quotes.
If you put a number in quotes, it will be treated as a text string.
One Statement, Many Variables
You can declare many variables in one statement.
Start the statement with let or
constand separate the variables by comma:
A declaration can span multiple lines:
The Assignment Operator
In JavaScript, the equal sign (=) is an assignment operator, not an
equal to operator.
This is different from algebra. The following does not make sense in algebra:
x = x + 5
In JavaScript, however, it makes perfect sense: it assigns the value of x + 5 to x.
(It calculates the value of x + 5 and puts the result into x. The value of x is incremented by 5.)
Note
The equal to operator is written like == in JavaScript.
JavaScript Arithmetic
As with algebra, you can do arithmetic with JavaScript variables, using
operators like = and +:
You can also add strings, but strings will be concatenated:
Note
If you put a number in quotes, the rest of the numbers will be treated as strings, and concatenated.
Video: JavaScript Variables
