Javascript - Scope Chain Augmentation


There are two primary types of execution contexts, global and function.

There are other ways to augment the scope chain.

Certain statements add a temporary scope to the front of the scope chain that is later removed after code execution.

There are two statements that can create new scope:

  • The catch block in a try-catch statement
  • A with statement

Both of these statements add a variable object to the front of the scope chain.

For the catch statement, a new variable object is created and contains a declaration for the thrown error object.

For the with statement, the specified object is added to the scope chain.

Consider the following:

function buildUrl() {
   var qs = "new value";

       var url = href + qs;

   return url;

In this example, the with statement is acting on the location object, so location itself is added to the front of the scope chain.

There is one variable, qs, defined in the buildUrl() function.

When the variable href is referenced, it's actually referring to location.href.

When the variable qs is referenced, it's referring to the variable defined in buildUrl(), which is in the function context's variable object.

Inside the with statement is a variable declaration for url, which becomes part of the function's context and can be returned as the function value.