Detect cyclic objects

- detect cyclic objects
- throw TypeError

json2.js

@@ -1,5 +1,5 @@
 //  json2.js
-//  2017-06-12
+//  2022-10-30
 //  Public Domain.
 //  NO WARRANTY EXPRESSED OR IMPLIED. USE AT YOUR OWN RISK.
 
@@ -231,6 +231,28 @@ if (typeof JSON !== "object") {
     }
 
 
+// This variable is initialized with an empty array every time
+// JSON.stringify() is invoked and checked by the str() function. It's
+// used to keep references to object structures and capture cyclic
+// objects. Every new object is checked for its existence in this
+// array. If it's found it means the JSON object is cyclic and we have
+// to stop execution and throw a TypeError accordingly the ECMA262
+// (see NOTE 1 by the link https://tc39.es/ecma262/#sec-json.stringify).
+
+    var seen;
+
+// Emulate [].includes(). It's actual for old-fashioned JScript.
+
+    function includes(array, value) {
+        for (var i = 0; i < array.length; i++) {
+            if (value === array[i]) {
+                return true;
+            }
+        }
+        return false;
+    }
+
+
     function str(key, holder) {
 
 // Produce a string from holder[key].
@@ -295,6 +317,16 @@ if (typeof JSON !== "object") {
                 return "null";
             }
 
+// Check the value is not circular object. Otherwise throw TypeError.
+
+            if (includes(seen, value)) {
+                throw new TypeError('Converting circular structure to JSON');
+            }
+
+// Keep the value for the further check on circular references.
+
+            seen.push(value);
+
 // Make an array to hold the partial results of stringifying this object value.
 
             gap += indent;
@@ -428,6 +460,10 @@ if (typeof JSON !== "object") {
                 throw new Error("JSON.stringify");
             }
 
+// Initialize the reference keeper.
+
+            seen = [];
+
 // Make a fake root object containing our value under the key of "".
 // Return the result of stringifying the value.