Add info about Variables in JavaScript Swap76#197

Author: criscros-garcia

docs/JavaScript_Basics/variables.md

@@ -1 +1,87 @@
-# Variables 
+# Variables
+
+## Datatypes
+One of the most fundamental characteristics of a programming language is the set of data types it
+supports. These are the type of values that can be represented and manipulated in a
+programming language.
+JavaScript allows you to work with three primitive data types −
+Numbers, eg. 123, 120.50 etc.
+Strings of text e.g. "This text string" etc.
+Boolean e.g. true or false.
+JavaScript also defines two trivial data types, null and undefined, each of which defines only a
+single value. In addition to these primitive data types, JavaScript supports a composite data type
+known as object. We will cover objects in detail in a separate chapter.
+Note − Java does not make a distinction between integer values and floating-point values. All
+numbers in JavaScript are represented as floating-point values. JavaScrip
+
+Like many other programming languages, JavaScript has variables. Variables can be thought of as
+named containers. You can place data into these containers and then refer to the data simply by
+naming the container.
+Before you use a variable in a JavaScript program, you must declare it. Variables are declared with
+the var keyword as follows.
+```js
+<script type="text/javascript">
+ var money;
+ var name;
+</script>
+```
+You can also declare multiple variables with the same var keyword as follows −
+```js
+<script type="text/javascript">
+<!--
+ var money, name;
+//-->
+</script>
+```
+Storing a value in a variable is called variable initialization. You can do variable initialization at
+the time of variable creation or at a later point in time when you need that variable.
+For instance, you might create a variable named money and assign the value 2000.50 to it later.
+For another variable, you can assign a value at the time of initialization as follows.
+```js
+<script type="text/javascript">
+<!--
+ var name = "Ali";
+ var money;
+ money = 2000.50;
+//-->
+</script>
+```
+Note − Use the var keyword only for declaration or initialization, once for the life of any variable
+name in a document. You should not re-declare same variable twice.
+JavaScript is untyped language. This means that a JavaScript variable can hold a value of any
+data type. Unlike many other languages, you don't have to tell JavaScript during variable
+declaration what type of value the variable will hold. The value type of a variable can change
+during the execution of a program and JavaScript takes care of it automatically.
+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.
+```js
+<script type="text/javascript">
+<!--
+ var myVar = "global"; // Declare a global variable
+ function checkscope( ) {
+ var myVar = "local"; // Declare a local variable
+ document.write(myVar);
+}
+//-->
+</script>
+```
+This produces the following result −
+local
+JavaScript Variable Names
+While naming your variables in JavaScript, keep the following rules in mind.
+You should not use any of the JavaScript reserved keywords as a variable name. These
+keywords are mentioned in the next section. For example, break or boolean variable names
+are not valid.
+JavaScript variable names should not start with a numeral 0 − 9. They must begin with a letter
+or an underscore character. For example, 123test is an invalid variable name but _123test
+is a valid one.
+JavaScript variable names are case-sensitive. For example, Name and name are two
+different variables.