What are Variables?
Variables are containers for storing data (storing data values).
In this example, x, y, and z,
are variables, declared with the var keyword:
Example
var x = 5;
var y = 6;
var z = x + y;
In this example, x, y, and z,
are variables, declared with the let keyword:
Example
let x = 5;
let y = 6;
let z = x + y;
In this example, x, y, and z,
are undeclared variables:
Example
x = 5;
y = 6;
z = x + y;
From all the examples above, you can guess:
- x stores the value 5
- y stores the value 6
- z stores the value 11
When to Use JavaScript var?
Always declare JavaScript variables with var,let,
orconst.
The var keyword is used in all JavaScript code from 1995 to 2015.
The let and const keywords were added to JavaScript in 2015.
If you want your code to run in older browser, you must use var.
When to Use JavaScript const?
If you want a general rule: always declare variables with const.
If you think the value of the variable can change, use let.
In this example, price1, price2, and total, are variables:
Example
const price1 = 5;
const price2 = 6;
let total = price1 + price2;
The two variables price1 and price2
are declared with the const keyword.
These are constant values and cannot be changed.
The variable total is declared with the let keyword.
This is a value that can be changed.
Just Like Algebra
Just like in algebra, variables hold values:
let x = 5;
let y = 6;
Just like in algebra, variables are used in expressions:
let z = x + y;
From the example above, you can guess that the total is calculated to be 11.
Note
Variables are containers for storing values.
JavaScript Identifiers
All JavaScript variables must be identified with unique names.
These unique names are called identifiers.
Identifiers can be short names (like x and y) or more descriptive names (age, sum, totalVolume).
The general rules for constructing names for variables (unique identifiers) are:
- Names can contain letters, digits, underscores, and dollar signs.
- Names must begin with a letter
- Names can also begin with $ and _ (but we will not use it in this tutorial)
- Names are case sensitive (y and Y are different variables)
- Reserved words (like JavaScript keywords) cannot be used as names
Note
JavaScript identifiers are case-sensitive.
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 Data Types
JavaScript variables can hold numbers like 100 and text values like "John Doe".
In programming, text values are called text strings.
JavaScript can handle many types of data, but for now, just think of numbers and strings.
Strings are written inside double or single quotes. Numbers are written without quotes.
If you put a number in quotes, it will be treated as a text string.
Example
const pi = 3.14;
let person = "John Doe";
let answer = 'Yes I am!';
Declaring a JavaScript Variable
Creating a variable in JavaScript is called "declaring" a variable.
You declare a JavaScript variable with the var or the let keyword:
var carName;
or:
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";
You can also assign a value to the variable when you declare it:
let carName = "Volvo";
In the example below, we create a variable called carName and assign the value
"Volvo" to it.
Then we "output" the value inside an HTML paragraph with id="demo":
Example
<p id="demo"></p>
<script>
let carName = "Volvo";
document.getElementById("demo").innerHTML = carName;
</script>
Note
It's a good programming practice to declare all variables at the beginning of a script.
One Statement, Many Variables
You can declare many variables in one statement.
Start the statement
with let and separate the variables by comma:
Example
let person = "John Doe", carName = "Volvo", price = 200;
A declaration can span multiple lines:
Example
let person = "John Doe",
carName = "Volvo",
price = 200;
Value = undefined
In computer programs, variables are often declared without a value. The value can be something that has to be calculated, or something that will be provided later, like user input.
A variable declared without a value will have the value
undefined.
The variable carName will have the value undefined after the execution of this statement:
Example
let carName;
Re-Declaring JavaScript Variables
If you re-declare a JavaScript variable declared with var, it will not lose its value.
The variable carName will still have the value "Volvo" after the execution of these statements:
Example
var carName = "Volvo";
var carName;
Note
You cannot re-declare a variable declared with let or const.
This will not work:
let carName = "Volvo";
let carName;
JavaScript Arithmetic
As with algebra, you can do arithmetic with JavaScript variables, using
operators like = and +:
Example
let x = 5 + 2 + 3;
You can also add strings, but strings will be concatenated:
Example
let x = "John" + " " + "Doe";
Also try this:
Example
let x = "5" + 2 + 3;
Note
If you put a number in quotes, the rest of the numbers will be treated as strings, and concatenated.
Now try this:
Example
let x = 2 + 3 + "5";
JavaScript Dollar Sign $
Since JavaScript treats a dollar sign as a letter, identifiers containing $ are valid variable names:
Example
let $ = "Hello World";
let $$$ = 2;
let $myMoney = 5;
Using the dollar sign is not very common in JavaScript, but professional programmers often use it as an alias for the main function in a JavaScript library.
In the JavaScript library jQuery, for instance, the main function
$ is used to select HTML elements.
In jQuery $("p"); means "select all p elements".
JavaScript Underscore (_)
Since JavaScript treats underscore as a letter, identifiers containing _ are valid variable names:
Example
let _lastName = "Johnson";
let _x = 2;
let _100 = 5;
Using the underscore is not very common in JavaScript, but a convention among professional programmers is to use it as an alias for "private (hidden)" variables.
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