JavaScript Scope

Scope determines if variables are accessible (visible).

A scope in JavaScript defines what variables you have access to. There are two kinds of scope – global scope and local scope.

Function scope and block scope are local scope - variables declared inside a function or a block is local to the function/block.


Global Scope and Local Scope

It is important to understand Global-Scope, Function-Scope and Block-Scope to know when you may use the different statements.

Global Scope

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

Be careful with using Global Scope.

var car = "Byton";
 
// code CAN use variable car
 
function carFunction() {
     // code CAN also use variable car 
}

Function Scope

Variables declared inside a function have Function Scope.

// code can NOT use variable car
 
function carFunction() {
     var car = "Byton";
     // code CAN use variable car
}
 
// code can NOT use variable car

Block Scope

Variables declared inside a block {} have Block Scope.

{ 
     var car = "Byton"; 
}
// code CAN use variable car

Variables that are not declared - Automatically Global

Variables will automatically become a GLOBAL variable if you assign a value to a variable that has not been declared.

JS JavaScript - Variables that are not declared - Automatically Global

carFunction();
 
// This code can use the variable carName
 
function carFunction() {
    carName = "Byton";
} 

Strict Mode

Modern browsers support running JavaScript in "Strict Mode".

In strict mode you can not, for example, use undeclared variables.



JavaScript Variables Lifetime

The lifetime of a JavaScript variable starts when it is declared.

Local variables are deleted when the function or block is completed.

In a web browser, global variables are deleted when you close the browser window.


Learn more about JavaScript Variables Learn more about JavaScript Comments
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JavaScript is one of the 3 core language technologies of the World Wide Web that all web developers must learn:

1.      HTML to define the content of web pages.

2.      CSS to specify the layout of web pages.

3.      JavaScript to program the behavior of web pages.

JavaScript is a scripting or programming language that allows you to implement complex things on web pages. Every time a web page does more than just sit there and display static information for you to look at, displaying timely content updates, interactive maps, animated 2D/3D graphics, scrolling video jukeboxes, etc. − you can bet that JavaScript is probably involved.


The benefits of using JavaScript are:

1.      You can validate user input before sending the page off to the server. This saves server traffic, which means less load on your server.

2.      Immediate feedback to the visitors − They don't have to wait for a page reload to see if they have forgotten to enter something.

3.      Increased interactivity − You can create interfaces that react when the user hovers over them with a mouse or activates them via the keyboard.

4.      Richer interfaces − You can use JavaScript to include such items as drag-and-drop components and sliders to give a Rich Interface to your site visitors.


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