JavaScript : Variables Declaring

[1]. 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 variables are containers for 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;

From the example above, you can expect:

• x stores the value 5
• y stores the value 6
• z stores the value 11

[2]. 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

[3].  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.

[3]. 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!';

[4]. Declaring (Creating) JavaScript Variables

Creating a variable in JavaScript is called "declaring" a variable.

You declare a JavaScript variable with the var keyword:

carName = "Volvo";

You can also assign a value to the variable when you declare it:

var 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>
var carName = "Volvo";
document.getElementById("demo").innerHTML = carName;
</script>

[5].  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;

[6]. 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

var carName;

[7]. 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

var carName = "Volvo";
var carName;

[8].   JavaScript Arithmetic

As with algebra, you can do arithmetic with JavaScript variables, using operators like = and +:

Example

var x = 5 + 2 + 3;

You can also add strings, but strings will be concatenated:

Example

var x = "John" + " " + "Doe";

Also try this:

Example

var x = "5" + 2 + 3;

If you put a number in quotes, the rest of the numbers will be treated as strings, and concatenated.

Example

var x = 2 + 3 + "5";

[9]. 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;

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".

[10].  JavaScript Underscore (_)

Since JavaScript treats underscore as a letter, identifiers containing _ are valid variable names:

Example

var _lastName = "Johnson";
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.