What is Closure ?

A closure is an inner function that has access to the variables in the outer (enclosing) function’s scope chain.

The closure has access to variables in three scopes; specifically:
(1) variable in its own scope,
(2) variables in the enclosing function’s scope, and
(3) global variables.

Here is an example :

var globalVar = "xyz";
(function outerFunc(outerArg) {
  var outerVar = 'a';
  (function innerFunc(innerArg) {
    var innerVar = 'b';
    console.log(
      "outerArg = " + outerArg + "\n" +
      "innerArg = " + innerArg + "\n" +
      "outerVar = " + outerVar + "\n" +
      "innerVar = " + innerVar + "\n" +
      "globalVar = " + globalVar);
  })(456);
})(123);

In the above example, variables from innerFunc, outerFunc, and the global namespace are all in scope in the innerFunc. The above code will therefore produce the following output:

outerArg = 123
innerArg = 456
outerVar = a
innerVar = b
globalVar = xyz