In this example,
z, are variables, declared with the
var y = 6;
var z = x + y;
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 (ES6)
Before 2015, using the
keyword to define a variable that cannot be reassigned, and
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
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,
total, are variables:
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.
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)
The Assignment Operator
=) is an "assignment" operator, not an
"equal to" operator.
This is different from algebra. The following does not make sense in algebra:
(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 programming, text values are called text 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.
var person = "John Doe";
var answer = 'Yes I am!';
After the declaration, the variable has no value (technically it has the
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 id="demo":
var carName = "Volvo";
document.getElementById("demo").innerHTML = carName;
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
var and separate the variables by comma:
A declaration can span multiple lines:
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
The variable carName will have the value
undefined after the execution of this statement:
carName will still have the value "Volvo" after the execution of these statements:
You can also add strings, but strings will be concatenated:
Also try this:
If you put a number in quotes, the rest of the numbers will be treated as strings, and concatenated.
Now try this:
- A letter (A-Z or a-z)
- A dollar sign ($)
- Or an underscore (_)
var $ = 2;
var $myMoney = 5;
$ is used to select HTML elements.
$("p"); means "select all p elements".
var _x = 2;
var _100 = 5;