Define “cyclic data structures”

Question

At the JSON site it says

JSON does not support cyclic data structures, so be careful to not give cyclical structures to the JSON stringifier.

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Answer 1

The object contains a cycle, i.e., it refers to itself or, more generally, some object to which it refers either directly or through some property is the original object.

 var $this =  { };
 $this["self"] = $this;

or more likely

 var child = { parent: null };
 var parent = { child: child };
 child.parent = parent;