JavaScript variables are containers for storing data values.
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In this example,
x
, y
, and z
, are variables, declared with the var
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Example
Try it Yourself »From the example above, you can expect:
- x stores the value 5
- y stores the value 6
- z stores the value 11
Using let and const (2015)
Before 2015, using the
var
keyword was the only way to declare a JavaScript variable.The 2015 version of JavaScript (ES6 - ECMAScript 2015) allows the use of the
const
keyword to define a variable that cannot be reassigned, and the let
keyword to define a variable with restricted scope.Because it is a little complicated to describe the difference between these keywords, and because they are not supported in older browsers, the first part of this tutorial will most often use
var
.Safari 10 and Edge 14 were the first browsers to fully support ES6:
Chrome 58 | Edge 14 | Firefox 54 | Safari 10 | Opera 55 |
Jan 2017 | Aug 2016 | Mar 2017 | Jul 2016 | Aug 2018 |
Much Like Algebra
In this example,
price1
, price2
, and total
, are variables:Example
var price1 = 5;
var price2 = 6;
var total = price1 + price2;
Try it Yourself »var price2 = 6;
var total = price1 + price2;
In programming, just like in algebra, we use variables (like price1) to hold values.
In programming, just like in algebra, we use variables in expressions (total = price1 + price2).
From the example above, you can calculate the total to be 11.
JavaScript variables are containers for storing data 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
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:
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.)
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
var pi = 3.14;
var person = 'John Doe';
var answer = 'Yes I am!';
Try it Yourself »var person = 'John Doe';
var answer = 'Yes I am!';
Declaring (Creating) JavaScript Variables
Creating a variable in JavaScript is called 'declaring' a variable.
You declare a JavaScript variable with the
var
keyword:After the declaration, the variable has no value (technically it has the value of
undefined
).To assign a value to the variable, use the equal sign:
You can also assign a value to the variable when you declare it:
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:
Example
<p></p>
<script>
var carName = 'Volvo';
document.getElementById('demo').innerHTML = carName;
</script>
Try it Yourself »<script>
var carName = 'Volvo';
document.getElementById('demo').innerHTML = carName;
</script>
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. Microsoft works database update.
Start the statement with
var
and separate the variables by comma: var person = 'John Doe', carName = 'Volvo', price = 200;
Try it Yourself »A declaration can span multiple lines:
var person = 'John Doe',
carName = 'Volvo',
price = 200;
Try it Yourself »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
Try it Yourself »Re-Declaring JavaScript Variables
If you re-declare a JavaScript variable, it will not lose its value.
The variable
carName
will still have the value 'Volvo' after the execution of these statements:Example
Try it Yourself »![Condenser Condenser](https://assets.ajmadison.com/image/upload/c_limit,f_auto,fl_lossy.progressive,h_1000,q_auto,w_1000/v1/ajmadison/images/large_no_watermark/wtb86200uc_300a4.jpg)
JavaScript Arithmetic
As with algebra, you can do arithmetic with JavaScript variables, using operators like
=
and +
: Example
Try it Yourself »You can also add strings, but strings will be concatenated:
Example
Try it Yourself »Also try this:
Example
Try it Yourself »If you put a number in quotes, the rest of the numbers will be treated as strings, and concatenated.
Now try this:
Example
Try it Yourself »JavaScript Dollar Sign $
Remember that JavaScript identifiers (names) must begin with:
- A letter (A-Z or a-z)
- A dollar sign ($)
- Or an underscore (_)
Since JavaScript treats a dollar sign as a letter, identifiers containing $ are valid variable names:
Example
var $$$ = 'Hello World';
var $ = 2;
var $myMoney = 5;
Try it Yourself »var $ = 2;
var $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:
Javascript Condenser 1 4.1
Example
var _lastName = 'Johnson';
var _x = 2;
var _100 = 5;
Try it Yourself »var _x = 2;
var _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.
You can test your JavaScript skills with W3Schools' Quiz.
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The Test
The test contains 25 questions and there is no time limit.
The test is not official, it's just a nice way to see how much you know, or don't know, about JavaScript.
Count Your Score
You will get 1 point for each correct answer. At the end of the Quiz, your total score will be displayed. Maximum score is 25 points.
Start the Quiz
Good luck!
If you don't know JavaScript, we suggest that you read our JavaScript Tutorial from scratch.
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