DataTypes and Scope In JavaScript

DataTypes and Scope In JavaScript
JavaScript is the most popular language In the world and it runs on the most popular environment means web(browser); By using JavaScript you can create complete web and desktop application.

DataTypes in Javascript:
In JavaScript, there are 3 types of primitive data-types.

1. Number
2. String
3. Boolean

1. Number: in this, you can assign values like 10,10.2 means even if you assign a floating-point number to a variable it is treated like a number data-types.
2. String: in this, you can assign a single character or multiple characters it is treated as string datatype.
3. Boolean: You can assign true and false value in this.
4. Undefined: in this, you declared a variable but you don’t assign the value to it is undefined.
5. Null: if a variable can not contain value other than null it is null.

Datatypes in JavaScript can be defined at run-time.

Suppose I declared a variable

1. Var x=10;

And after that, I assign string value like this x=”sagar” so at run-time if you use like below you will know the data types are changed.

Code:

var x=10; 
[LIST=1]
console.log(typeof(x));//number 
x="sagar"; 
console.log(typeof(x)); //string
  

[/LIST]

1. 
   

Points to remember:

1. Datatypes are not strongly typed like java, c# language.
2. Whatever value you assigned of right-hand side the left-hand side variable becomes of that data-type.

Scope In JavaScript:
In JavaScript, we have only two scopes Private and Global. In strongly typed language we have a different type of scopes like public, private, protected,internal like that but in JavaScript we have only 2 scopes Private and Global.

Code:

var x=10; // global scope 
console.log(x); //output-10 
function PrintX() 
{ 
var x=12; //private scope 
console.log(x); //output-12 
} 
PrintX(); 
 

Global Scope In JavaScript
In this above example, you can see x variable which declared outside the function Printx is treated as a global variable. And this global variable access throughout the page or that respective javascript file.

Private: In this, the variable which is declared inside the function is treated as a private variable and it is not accessed outside that function see below example

Code:

function PrintX() 
{ 
var x=12; //private scope 
console.log(x); //output-12 
} 
console.log(x); 
PrintX(); 
 

In this example, if run this code it throws an error.

ReferenceError: x is not defined
which is a reference error.

To decide the scope of variable in JavaScript, it uses lexical scope approach. So what is meaning lexical approach it says that depending on the position of a word the meaning of word change. In JavaScript depending on the position left or right the scope will change means whether it is inside the function or outside of a function.

Auto global variable or scope:
In JavaScript when you didn’t declare the variable and assign value to that variable it will become a global variable.

Code:

function display() { 
x = 10; 
console.log(x); //output-10 
} 
display(); 
console.log(x);////output-10 
 

Use Strict:
But to avoid this functionality you can use “use strict”. So what is the use of use strict is that you can’t use a variable before declaration means by using this “use strict” we can raise an error if we use a variable before declaration.

Code:

"use strict" 
function display() { 
x = 10; 
console.log(x); //output-10 
} 
display(); 
console.log(x);////output-10 
 

the out of this code is as below which throws an error.

1. index.htm:13 Uncaught ReferenceError: x is not defined
2. at display (index.htm:13)
3. at index.htm:17