Variables are used for storing data values. JavaScript variables are containers (placeholders) for storing values. A variable can have different values at different phases of execution. Since the content of a variable varies throughout the execution of a program, they are called variables.
Below example, x, y, and z, are variables:
<!DOCTYPE html>
<html>
<body>
<h1>JavaScript Variables</h1>
<p>In below example, num1, num2, and sum are variables</p>
var num1 = 25;<br />
var num2 = 75;<br />
var sum = num1 + num2; // sum is 100<br /><br />
Sum : <span id="element"></span>
<script>
var num1 = 25;
var num2 = 75;
var sum = num1 + num2; // sum is 100
document.getElementById("element").innerHTML = sum;
</script>
</body>
</html>
As per above example
Javascript variables must be unique and identified by unique names. These unique variable names are called identifiers. Identifiers can be of single character (a, b, c) or with multiple characters for descriptive names (age, name, person). You can’t use a JavaScript built in keyword (such as var, if, else) as identifiers.
General programming rules are applied for constructing variables:
Creating a variable in any programming language is called “declaring” a variable. JavaScript variable can be declared with the var keyword:
var locationName;
After declaration, the variable has no value. It has default value of undefined. To assign a value to the variable, use the equal sign.
locationName = "Park Grove, TX";
It is also possible to assign value to a variable during declaration
var locationName = "La Cross Road";
It is possible to declare many variables using single var keywords. Start with var keyword and than separate each variable with comma. In below example 3 variables are declared in single statement.
<!DOCTYPE html>
<html>
<body>
<h1>JavaScript Variables</h1>
<p>You can declare many variables in one statement.</p>
var personName = "Keyur Gohel", locationName = "Ispat Bhavan, Lodhi Road", price = 200; <br /><br />
value of locationName variable is <span id="element"></span>
<script>
var personName = "Keyur Gohel", locationName = "Ispat Bhavan, Lodhi Road", price = 200;
document.getElementById("element").innerHTML = locationName;
</script>
</body>
</html>
It is possible to declare variable without value. Variable declared without any value will have undefined value. The variable location will have the value undefined after the execution of this statement:
<!DOCTYPE html>
<html>
<body>
<h1>JavaScript Variables</h1>
<p>When a variable is declared without a value, it will have the value undefined.</p>
var locationName;<br /><br />
here locationName variable is set value of <span id="demo"></span>
<script>
var locationName;
document.getElementById("demo").innerHTML = locationName;
</script>
</body>
</html>