Scope is the set of variables, objects, and functions you have access to.

JavaScript has two scopes: global and local. A variable that is declared outside a function definition is a global variable, and its value is accessible and modifiable throughout your program. A variable that is declared inside a function definition is local. It is created and destroyed every time the function is executed, and it cannot be accessed by any code outside the function.

Functions defined inside other functions, known as nested functions, have access to their parent function's scope.

Pay attention to the comments in the code below:

var a = 4;	// a is a global variable, it can be accessed by the functions below

function foo() {
	var b = a * 3;	// b cannot be accessed outside foo function, but can be accessed by functions
					// defined inside foo
	function bar(c) {
	var b = 2;  // another `b` variable is created inside bar function scope
				// the changes to this new `b` variable don't affect the old `b` variable
	console.log( a, b, c );
	}

	bar(b * 4);
}

foo(); // 4, 2, 48

IIFE, Immediately Invoked Function Expression, is a common pattern for creating local scopes example:

	(function(){ // the function expression is surrounded by parenthesis
		// variables defined here
		// can't be accessed outside
	})(); // the function is immediately invoked

Create a file named scope.js.

In that file, copy the following code:

var a = 1, b = 2, c = 3;

(function firstFunction(){
	var b = 5, c = 6;

	(function secondFunction(){
		var b = 8;

		(function thirdFunction(){
			var a = 7, c = 9;

			(function fourthFunction(){
				var a = 1, c = 8;

			})();
		})();
	})();
})();

Use your knowledge of the variables' scope and place the following code inside one of the functions in scope.js so the output is a: 1, b: 8, c: 6

console.log("a: "+a+", b: "+b+", c: "+c);

Check to see if your program is correct by running this command:

javascripting verify scope.js