JavaScript Scope

JavaScript Scope

The scope of a variable is region of your program in which it is defined. JavaScript will have only to scopes. 

Scope- The scope of a variable is region of your program in which it is defined. JavaScript will have only to scopes. the enclosing context values and expressions are associated. 

• essentially, the visibility of various identifiers in a program

Why scope 

• Controls the visibility of variables and parameters.
• A programmer should know about scope to prevent naming collisions and ambiguity.

Why scope in JavaScript 

• JavaScript is NOT “some other language”
• Unlike other languages like C, C++ JavaScript doesn’t support block level scope
• Supports function level scope

There are three types of scope in JavaScript:

• Block scope
• Function scope
• Global scope

Block scope

A block scoped variable means that the variable defined within a block will not be accessible from outside the block. A block can reside inside a function, and a block scoped variable will not be available outside the block even if the block is inside a function.

Example

For (var i = 0; i <=0; i++) {

    // block code

} 

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

Function Scope

In Function Scope each function creates a new scope.

 In Function scope variables defined inside a function are not accessible (visible) from outside the function.

In Function Scope variables declared with var, let and const are quite similar when declared inside a function.

Example

function vehicle() {
  var vehicalName = "car";   // Function Scope
}

Global Scope

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

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

Example

var x = 2;       // Global scope