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What they have in common: they are all primitive types and are stored locally in variables in the stack.

Differences:

(1) undefined-- indicates that the variable is declared but not assigned a value.

It is the default value for all unassigned variables, for example:

Vara;//an is automatically assigned to undefined

(2) null-- indicates that a variable may point to an object in the future.

Typically used to actively release references to objects, such as:

Varemps= ['ss','nn']

Emps=null;// releases references to arrays

Extension-refuse collection bin

It is a program that specializes in freeing object memory.

(1) at the bottom, the background runs with the current program at the same time; the engine will call the garbage collection period automatically.

(2) there is always an object that is no longer referenced by any variable.

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