Professional.JavaScript.for.Web.Developers.3rd.Edition.Jan.2012
JavaScript is a garbage-collected language, meaning that the execution environment is responsible
for managing the memory required during code execution. In languages like C and C++, keeping
track of memory usage is a principle concern and the source of many issues for developers.
JavaScript frees developers from worrying about memory management by automatically allocating
what is needed and reclaiming memory that is no longer being used. The basic idea is simple: figure
out which variables aren’t going to be used and free the memory associated with them. This process
is periodic, with the garbage collector running at specified intervals (or at predefined collection
moments in code execution).
Consider the normal life cycle of a local variable in a function. The variable comes into existence
during the execution of the function. At that time, memory is allocated on the stack (and possibly
on the heap) to provide storage space for the value. The variable is used inside the function and then
the function ends. At that point this variable is no longer needed, so its memory can be reclaimed
for later use. In this situation, it’s obvious that the variable isn’t needed, but not all situations are
as obvious. The garbage collector must keep track of which variables can and can’t be used so it
can identify likely candidates for memory reclamation. The strategy for identifying the unused
variables may differ on an implementation basis, though two strategies have traditionally been used
in browsers.
//局部变量只在函数执行的过程中存在。而在这个过程中，会为局部变量在栈（或堆）内存上分配相应的空间，以便存储它们的值。然后在函数中使用这些变量，直至函数执行结束。