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DataTypes and Scope In JavaScript

Discussion in 'JavaScript and AJAX' started by Sagar Jaybhay, Sep 20, 2019.

  1. Sagar Jaybhay

    Sagar Jaybhay New Member

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    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]

    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
     
    shabbir likes this.

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