A brief introduction to JavaScript variables and data types_Basic knowledge

JavaScript data type:

One of the most fundamental features of a programming language is the set of data types it supports. These are the types of programming language values ​​that can be represented and manipulated.

JavaScript allows three basic data types:

1. Numbers such as 123, 120.50, etc.
2. Strings such as "This text string" etc.
3. Boolean type, such as true or false.

JavaScript also defines two data types: null and undefined, each of which only limits a single value.

In addition to these basic data types, JavaScript supports composite data types called objects. We'll see object details covered in a separate chapter.

Note: Java does not make a distinction between integer and floating point values. All numbers in JavaScript are represented as floating point values. JavaScript represents numbers using the 64-bit floating point format defined by the IEEE 754 standard.
JavaScript variables:

Like many other programming languages, JavaScript has variables. Variables can be thought of as named containers. You can put data into these containers and then simply name the containers with reference to that data.

To use a variable in a JavaScript program, it must be declared. Variables are declared using the var keyword as follows:

<script type="text/javascript">
<!--
var money;
var name;
//-->
</script>

You can also declare multiple variables with the same var keyword as follows:

<script type="text/javascript">
<!--
var money, name;
//-->
</script>

The value stored in a variable is called the initialization of the variable. It can be initialized when a variable is created or updated. It requires a variable, as shown below:

For example, you can create a variable named money and a value of 2000.50, and then assign it to it. For another variable, you can assign a value when initializing as follows:

<script type="text/javascript">
<!--
var name = "Ali";
var money;
money = 2000.50;
//-->
</script>

Note: Use the var keyword only for declaration or initialization. Once a variable name is declared it lives throughout the document. No need to redeclare the same variable twice.

JavaScript is a typed language. This means that JavaScript variables can hold values ​​of any data type. Unlike many other languages, you don't have to tell JavaScript in a variable declaration what type of value the variable will hold. The value type of a variable can be changed during the execution of the program and JavaScript will do so automatically.
JavaScript variable scope:

The scope of a variable is the area in the program where it is defined. JavaScript variables will only have two categories.

• Global variables: Global variables have global scope, which means that they are defined everywhere in JavaScript code.
• Local variables: A local variable will only be visible in the function where it is defined. Function parameters are local functions.

In the body of a function, local variables take precedence over global variables with the same name. If you declare a global variable with the same name as a local variable or a function parameter, you can effectively hide the global variable. Let’s give an example below:

<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 will produce the following results:

local

JavaScript variable name:

Although variable naming in JavaScript maintains the following rules.

• You should not use any reserved JavaScript keywords as variable names. 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 number (0-9). They must be lettered or underlined. 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.

JavaScript reserved words:

The following are reserved words in JavaScript. They cannot be used with JavaScript variables, functions, methods, loop tags, or any object names.