add Garbage collection resource

Author: Bunlong

images/family-delete-refs.png

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+# Garbage collection
+
+Memory management in JavaScript is performed automatically and invisibly to us. We create primitives, objects, functions… All that takes memory.
+
+What happens when something is not needed any more? How does the JavaScript engine discover it and clean it up?
+
+## Reachability
+
+The main concept of memory management in JavaScript is reachability.
+
+Simply put, “reachable” values are those that are accessible or usable somehow. They are guaranteed to be stored in memory.
+
+1. There’s a base set of inherently reachable values, that cannot be deleted for obvious reasons.
+
+  For instance:
+
+  * Local variables and parameters of the current function.
+
+  * Variables and parameters for other functions on the current chain of nested calls.
+
+  * Global variables.
+
+  * (there are some other, internal ones as well)
+
+  These values are called roots.
+
+2. Any other value is considered reachable if it’s reachable from a root by a reference or by a chain of references.
+
+  For instance, if there’s an object in a local variable, and that object has a property referencing another object, that object is considered reachable. And those that it references are also reachable. Detailed examples to follow.
+
+There’s a background process in the JavaScript engine that is called [garbage](https://en.wikipedia.org/wiki/Garbage_collection_(computer_science) "garbage") collector. It monitors all objects and removes those that have become unreachable.
+
+## A simple example
+
+Here’s the simplest example:
+
+```
+// user has a reference to the object
+let user = {
+  name: "John"
+};
+```