JavaScript Variables
JavaScript Where To
JavaScript variables are containers for storing data values. In this example, x, y, and z, are variables, declared with the var keyword:
var x = 5;
var y = 6;
var z = x + y;
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:
Much Like Algebra
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 below, you can calculate the total to be 11.
var price1 = 5;
var price2 = 6;
var total = price1 + price2;
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: 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.) 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.
var pi = 3.14;
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: var carName;
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: carName = "Volvo";
It's a good programming practice to declare all variables at the beginning of a script.
<p id="demo"></p>
<script>
var carName = "Volvo";
document.getElementById("demo").innerHTML = carName;
</script>
One Statement, Many Variables
You can declare many variables in one statement. Start the statement with var and separate the variables by comma: var person = "John Doe", carName = "Volvo", price = 200;
A declaration can span multiple lines:
var 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.
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:
var carName = "Volvo";
var carName;
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:
var x = "John" + " " + "Doe";
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:
var $$$ = "Hello World";
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 Variable Scope
The scope of a variable is the region of your program in which it is defined. JavaScript variables have only two scopes.
- Global Variables − A global variable has global scope which means it can be defined anywhere in your JavaScript code.
- Local Variables − A local variable will be visible only within a function where it is defined. Function parameters are always local to that function.
Within the body of a function, a local variable takes precedence over a global variable with the same name. If you declare a local variable or function parameter with the same name as a global variable, you effectively hide the global variable. Take a look into the following example.
<html>
<body onload = checkscope();>
<script type = "text/javascript">
var myVar = "global"; // Declare a global variable
function checkscope( ) {
var myVar = "local"; // Declare a local variable
document.write(myVar);
}
</script>
</body>
</html>
JavaScript Underscore (_)
Since JavaScript treats underscore as a letter, identifiers containing _ are valid variable names.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.
var _lastName = "Johnson";
var _x = 2;
var _100 = 5;
