Variable Scope in javascript

 

 

 

Scope determines the accessibility (visibility) of variables.

JavaScript has 3 types of scope:

• Block scope
• Function scope
• Global scope
  

   

  1. Block Scope :
  

  Before ES6 (2015), JavaScript had only Global Scope and Function Scope.

  ES6 introduced two important new JavaScript keywords: let and const.

  These two keywords provide Block Scope in JavaScript.

   ---------------------------------------------------------------------------------------------

  Variables declared inside a { } block cannot be accessed from outside the block:

  Example:

  {
    let x = 2;
  }
  // x can NOT be used here  

   ----------------------------------------------------------------------------------------------

  Variables declared with the var keyword can NOT have block scope.

  Variables declared inside a { } block can be accessed from outside the block. 

  Example:

  {
    var x = 2;
  }
  // x CAN be used here  

   

   
  

  note that : 

  curly braces block { } like : if , switch  , for .....
  

  ______________________________________________________________

   

  2. Function Scope (Local Scope) :
  

  Variables declared within a JavaScript function, become LOCAL to the function.

  JavaScript has function scope: Each function creates a new scope.

  Variables defined inside a function are not accessible (visible) from outside the function.

  Variables declared with var, let and const are quite similar when declared inside a function.

  They all have Function Scope:

   

   

  Example:

  // code here can NOT use localVariable
  
  function myFunction() {
    let localVariable= "Hend";
    // code here CAN use localVariable
   }
  
  // code here can NOT use localVariable

   ---------------------------------------------------------------------------------------------------

   

  3. Global Scope :
  

  Variables declared Globally (outside any function) have Global Scope.

  Global variables can be accessed from anywhere in a JavaScript program.

  Variables declared with var, let and const are quite similar when declared outside a block ,They all have Global Scope.

  ---------------------------------------------------------------------------------------------------- 

  Automatically Global

  If you assign a value to a variable that has not been declared, it will automatically become a GLOBAL variable.

  This code example will declare a global variable myName, even if the value is assigned inside a function.

  Example:

  myFunction();
  
  // code here can use myName
  
  function myFunction() {
      myName= "Hend";
  }

   // code here can use myName