Javascript uses scope chains to establish the scope for a given function. There is typically one global scope, and each function defined has its own nested scope. Any function defined within another function has a local scope which is linked to the outer function. It's always the position in the source that defines the scope.

An element in the scope chain is basically a Map with a pointer to its parent scope.

When resolving a variable, javascript starts at the innermost scope and searches outwards.

Variables declared globally have a global scope. Variables declared within a function are scoped to that function, and shadow global variables of the same name.

(I'm sure there are many subtleties that real JavaScript programmers will be able to point out in other answers. In particular I came across this page about what exactly this means at any time. Hopefully this more introductory link is enough to get you started though.)

Here's an example:

<script>

var globalVariable = 7; //==window.globalVariable

function aGlobal( param ) { //==window.aGlobal(); 
                            //param is only accessible in this function
  var scopedToFunction = {
    //can't be accessed outside of this function

    nested : 3 //accessible by: scopedToFunction.nested
  };

  anotherGlobal = {
    //global because there's no `var`
  }; 

}

</script>

You'll want to investigate closures, and how to use them to make private members.

The key, as I understand it, is that Javascript has function level scoping vs the more common C block scoping.

Here is a good article on the subject.

Old school JavaScript

Traditionally, JavaScript really only has two types of scope :

1. Global Scope : Variables are known throughout the application, from the moment they are declared onwards
2. Functional Scope : Variables are known within the function they are declared in, from the moment they are declared onwards

I will not elaborate on this, since there are already many other answers explaining the difference.

Modern JavaScript

The most recent JavaScript specs now also allow a third scope :

3. Block Scope : Variables are known within the block they are declared in, from the moment they are declared onwards

How do I create block scope variables?

Traditionally, you create your variables like this :

var myVariable = "Some text";

Block scope variables are created like this :

let myVariable = "Some text";

So what is the difference between functional scope and block scope?

To understand the difference between functional scope and block scope, consider the following code :

// i IS NOT known here
// j IS NOT known here

function loop(arr) {
    // i IS NOT known here
    // j IS NOT known here

    for( var i = 0; i < arr.length; i++ ) {
        // i IS known here
        // j IS NOT known here
    };

    // i IS known here
    // j IS NOT known here

    for( let j = 0; j < arr.length; j++ ) {
        // i IS known here
        // j IS known here
    };

    // i IS known here
    // j IS NOT known here
}

// i IS NOT known here
// j IS NOT known here

Here, we can see that our variable j is only known in the first for loop, but not before and after. Yet, our variable i is known in the entire function from the moment it is defined onward.

Is it safe to use block scope variables today?

Whether or not it is safe to use today, depends on your environment :

• If you're writing server-side JavaScript code (Node.js), you can safely use the let statement.

• If you're writing client-side JavaScript code and use a transpiler (like Traceur), you can safely use the let statement, however your code is likely to be anything but optimal with respect to performance.

• If you're writing client-side JavaScript code and don't use a transpiler, you need to consider browser support.

  Today, Feb 23 2016, these are some browsers that either don't support let or have only partial support :

  • Internet explorer 10 and below (no support)
  • Firefox 43 and below (no support)
  • Safari 9 and below (no support)
  • Opera Mini 8 and below (no support)
  • Android browser 4 and below (no support)
  • Opera 36 and below (partial support)
  • Chome 51 and below (partial support)

How to keep track of browser support

For an up-to-date overview of which browsers support the let statement at the time of your reading this answer, see this Can I Use page.

In "Javascript 1.7" (Mozilla's extension to Javascript) one can also declare block-scope variables with let statement:

 var a = 4;
 let (a = 3) {
   alert(a); // 3
 }
 alert(a);   // 4

1) There is a global scope, a function scope, and the with and catch scopes. There is no 'block' level scope in general for variable's -- the with and the catch statements add names to their blocks.

2) Scopes are nested by functions all the way to the global scope.

3) Properties are resolved by going through the prototype chain. The with statement brings object property names into the lexical scope defined by the with block.

EDIT: ECMAAScript 6 (Harmony) is spec'ed to support let, and I know chrome allows a 'harmony' flag, so perhaps it does support it..

Let would be a support for block level scoping, but you have to use the keyword to make it happen.

EDIT: Based on Benjamin's pointing out of the with and catch statements in the comments, I've edited the post, and added more. Both the with and the catch statements introduce variables into their respective blocks, and that is a block scope. These variables are aliased to the properties of the objects passed into them.

 //chrome (v8)

 var a = { 'test1':'test1val' }
 test1   // error not defined
 with (a) { var test1 = 'replaced' }
 test1   // undefined
 a       // a.test1 = 'replaced'

EDIT: Clarifying example:

test1 is scoped to the with block, but is aliased to a.test1. 'Var test1' creates a new variable test1 in the upper lexical context (function, or global), unless it is a property of a -- which it is.

Yikes! Be careful using 'with' -- just like var is a noop if the variable is already defined in the function, it is also a noop with respect to names imported from the object! A little heads up on the name already being defined would make this much safer. I personally will never use with because of this.

The idea of scoping in JavaScript when originally designed by Brendan Eich came from the HyperCard scripting language HyperTalk.

In this language, the displays were done similar to a stack of index cards. There was a master card referred to as the background. It was transparent and can be seen as the bottom card. Any content on this base card was shared with cards placed on top of it. Each card placed on top had its own content which took precedence over the previous card, but still had access to the prior cards if desired.

This is exactly how the JavaScript scoping system is designed. It just has different names. The cards in JavaScript are known as Execution Contexts^ECMA. Each one of these contexts contains three main parts. A variable environment, a lexical environment, and a this binding. Going back to the cards reference, the lexical environment contains all of the content from prior cards lower in the stack. The current context is at the top of the stack and any content declared there will be stored in the variable environment. The variable environment will take precedence in the case of naming collisions.

The this binding will point to the containing object. Sometimes scopes or execution contexts change without the containing object changing, such as in a declared function where the containing object may be window or a constructor function.

These execution contexts are created any time control is transferred. Control is transferred when code begins to execute, and this is primarily done from function execution.

So that is the technical explanation. In practice, it is important to remember that in JavaScript

• Scopes are technically "Execution Contexts"
• Contexts form a stack of environments where variables are stored
• The top of the stack takes precedence (the bottom being the global context)
• Each function creates an execution context (but not always a new this binding)

Applying this to one of the previous examples (5. "Closure") on this page, it is possible to follow the stack of execution contexts. In this example there are three contexts in the stack. They are defined by the outer context, the context in the immediately invoked function called by var six, and the context in the returned function inside of var six's immediately invoked function.

i) The outer context. It has a variable environment of a = 1
ii) The IIFE context, it has a lexical environment of a = 1, but a variable environment of a = 6 which takes precedence in the stack
iii) The returned function context, it has a lexical environment of a = 6 and that is the value referenced in the alert when called.

I found that many people new to JavaScript have trouble understanding that inheritance is available by default in the language and that function scope is the only scope, so far. I provided an extension to a beautifier I wrote at the end of last year called JSPretty. The feature colors function scope in the code and always associates a color to all variables declared in that scope. Closure is visually demonstrated when a variable with a color from one scope is used in a different scope.

Try the feature at:

• http://prettydiff.com/jspretty.xhtml?c=white&jsscope

See a demo at:

• http://prettydiff.com/jspretty.xhtml?c=white&jsscope&s=http://prettydiff.com/lib/markup_beauty.js

View the code at:

• http://prettydiff.com/lib/jspretty.js
• https://github.com/austincheney/Pretty-Diff/blob/master/lib/jspretty.js

Currently the feature offers support for a depth of 16 nested functions, but currently does not color global variables.

JavaScript have only two type of scope :

1. Global Scope : Global is nothing but a window level scope.Here, variable present throughout the application.
2. Functional Scope : Variable declared within a function with var keyword has functional scope.

Whenever a function is called, a variable scope object is created (and included in scope chain) which is followed by variables in JavaScript.

        a = "global";
         function outer(){ 
              b = "local";
              console.log(a+b); //"globallocal"
         }
outer();

Scope chain -->

1. Window level - a and outer function are at top level in scope chain.
2. when outer function called a new variable scope object(and included in scope chain) added with variable b inside it.

Now when a variable a required it first searches for nearest variable scope and if variable is not there than it move's to next object of variable scope chain.which is in this case is window level.

run the code. hope this will give an idea about scoping

Name = 'global data';
document.Name = 'current document data';
(function(window,document){
var Name = 'local data';
var myObj = {
    Name: 'object data',
    f: function(){
        alert(this.Name);
    }
};

myObj.newFun = function(){
    alert(this.Name);
}

function testFun(){
    alert("Window Scope : " + window.Name + 
          "\nLocal Scope : " + Name + 
          "\nObject Scope : " + this.Name + 
          "\nCurrent document Scope : " + document.Name
         );
}


testFun.call(myObj);
})(window,document);

Global Scope :

Global variables are exactly like global stars (Jackie Chan, Nelson Mandela). You can access them (get or set the value), from any part of your application. Global functions are like global events (New Year, Christmas). You can execute (call) them from any part of your application.

//global variable
var a = 2;

//global function
function b(){
   console.log(a);  //access global variable
}

Local Scope :

If you are in the USA, you may know Kim Kardashian, infamous celebrity ( she somehow manages to make the tabloids). But people outside of the USA will not recognize her. She is a local star, bound to her territory.

Local variables are like local stars. You can only access them (get or set the value) inside the scope. A local function is like local events - you can execute only (celebrate) inside that scope. If you want to access them from outside of the scope, you will get a reference error

function b(){
   var d = 21; //local variable
   console.log(d);

   function dog(){  console.log(a); }
     dog(); //execute local function
}

 console.log(d); //ReferenceError: dddddd is not defined    

Check this article for in-depth understanding of scope

There are only function scopes in JS. Not block scopes! You can see what is hoisting too.

var global_variable = "global_variable";
var hoisting_variable = "global_hoist";

// Global variables printed
console.log("global_scope: - global_variable: " + global_variable);
console.log("global_scope: - hoisting_variable: " + hoisting_variable);

if (true) {
    // The variable block will be global, on true condition.
    var block = "block";
}
console.log("global_scope: - block: " + block);

function local_function() {
    var local_variable = "local_variable";
    console.log("local_scope: - local_variable: " + local_variable);
    console.log("local_scope: - global_variable: " + global_variable);
    console.log("local_scope: - block: " + block);
    // The hoisting_variable is undefined at the moment.
    console.log("local_scope: - hoisting_variable: " + hoisting_variable);

    var hoisting_variable = "local_hoist";
    // The hoisting_variable is now set as a local one.
    console.log("local_scope: - hoisting_variable: " + hoisting_variable);
}

local_function();

// No variable in a separate function is visible into the global scope.
console.log("global_scope: - local_variable: " + local_variable);

There are ALMOST only two types of JavaScript scopes:

• the scope of each var declaration is associated with the most immediately enclosing function
• if there is no enclosing function for a var declaration, it is global scope

So, any blocks other than functions do not create a new scope. That explains why for-loops overwrite outer scoped variables:

var i = 10, v = 10;
for (var i = 0; i < 5; i++) { var v = 5; }
console.log(i, v);
// output 5 5

Using functions instead:

var i = 10, v = 10;
$.each([0, 1, 2, 3, 4], function(i) { var v = 5; });
console.log(i,v);
// output 10 10

In the first example, there was no block scope, so the initially declared variables were overwritten. In the second example, there was a new scope due to the function, so the initially declared variables were SHADOWED, and not overwritten.

That's almost all you need to know in terms of JavaScript scoping, except:

• try/catch introduce new scope ONLY for the exception variable itself, other variables do not have new scope
• with-clause apparently is another exception, but using with-clause it highly discouraged (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/with)

So you can see JavaScript scoping is actually extremely simple, albeit not always intuitive. A few things to be aware of:

• var declarations are hoisted to the top of the scope. This means no matter where the var declaration happens, to the compiler it is as if the var itself happens at the top
• multiple var declarations within the same scope are combined

So this code:

var i = 1;
function abc() {
  i = 2;
  var i = 3;
}
console.log(i);     // outputs 1

is equivalent to:

var i = 1;
function abc() {
  var i;     // var declaration moved to the top of the scope
  i = 2;
  i = 3;     // the assignment stays where it is
}
console.log(i);

This may seem counter intuitive, but it makes sense from the perspective of a imperative language designer.

Just to add to the other answers, scope is a look-up list of all the declared identifiers (variables), and enforces a strict set of rules as to how these are accessible to currently executing code. This look-up may be for the purposes of assigning to the variable, which is an LHS (lefthand-side) reference, or it may be for the purposes of retrieving its value, which is an RHS (righthand-side) reference. These look-ups are what the JavaScript engine is doing internally when it's compiling and executing the code.

So from this perspective, I think that a picture would help that I found in the Scopes and Closures ebook by Kyle Simpson:

Quoting from his ebook:

The building represents our program’s nested scope ruleset. The first floor of the building represents your currently executing scope, wherever you are. The top level of the building is the global scope. You resolve LHS and RHS references by looking on your current floor, and if you don’t find it, taking the elevator to the next floor, looking there, then the next, and so on. Once you get to the top floor (the global scope), you either find what you’re looking for, or you don’t. But you have to stop regardless.

One thing of note that is worth mentioning, "Scope look-up stops once it finds the first match".

This idea of "scope levels" explains why "this" can be changed with a newly created scope, if it's being looked up in a nested function. Here is a link that goes into all these details, Everything you wanted to know about javascript scope

Try this curious example. In the example below if a were a numeric initialized at 0, you'd see 0 and then 1. Except a is an object and javascript will pass f1 a pointer of a rather than a copy of it. The result is that you get the same alert both times.

var a = new Date();
function f1(b)
{
    b.setDate(b.getDate()+1);
    alert(b.getDate());
}
f1(a);
alert(a.getDate());