Lexical Scope

There are generally two scope models:

• Lexical Scope,
• Dynamic Scope.

Javascript employs the lexical scope model.

Lexical scope implies that the scope is defined by author time decisions of where functions are declared.

Scope lookups for identifiers/variables begin at the inner-most scope and works its way outward the scope until finds the first match.

Some mechanisms like eval (...) and with can be used to ‘cheat’ lexical scope in such a way that lexical scope can be modified (or even created) at code runtime instead of the ‘ideal’ author-time.

eval(...) modifies an existing lexical scope at runtime while with can create a whole new lexical scope at runtime.

Using any of the aforementioned mechanisms to ‘cheat’ lexical scope generally impairs the Javascript engine’s ability to perform complile-time optimizations hence reducing the code’s performance.

Both of the ‘cheating’ mechanisms donot ‘cheat’ in Javascript’s “strict mode” and due to the perfomance issues presented, they should not be used when crafting code.