javascript

Chapter 4: Variables, Scope, and Memory
All variables, primitive and reference, exist within an execution context (also called a scope), that
determines the lifetime of the variable and which parts of the code can access it. Execution context can be
summarized as follows:
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Execution contexts exist globally (called the global context) as well as within functions.
Each time a new execution context is entered, it creates a scope chain to search for variables and
functions.
Contexts that are local to a function have access not only to variables in that scope, but also to
variables in any containing contexts as well as the global context.
The global context has access only to variables and functions in the global context and cannot
directly access any data inside local contexts.
The execution context of variables helps to determine when memory will be freed.
JavaScript is a garbage - collected programming environment where the developer need not be concerned
with memory allocation or reclamation. JavaScript ’ s garbage - collection routine can be summarized as
follows:
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Values that go out of scope will automatically be marked for reclamation and will be deleted
during the garbage - collection process.
The predominant garbage - collection algorithm is called mark - and - sweep, which marks values
that aren ’ t currently being used and then goes back to reclaim that memory.
Another algorithm is reference counting, which keeps track of how many references there are to
a particular value. JavaScript engines no longer use this algorithm, but it still affects IE due to
non - native JavaScript objects (such as DOM elements) being accessed in JavaScript.
Reference counting causes problems when circular references exist in code.
Dereferencing variables helps not only with circular references but also with garbage collection
in general. To aid in memory reclamation, global objects, properties on global objects, and
circular references should all be dereferenced when no longer needed.
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