Detecting and fixing circular references in JavaScript

Question

Given I have a circular reference in a large JavaScript object

And I try JSON.stringify(problematicObject)

And the browser throws

Answer 1

There's a lot of answers here, but I thought I'd add my solution to the mix. It's similar to @Trey Mack's answer, but that solution takes O(n^2). This version uses WeakMap instead of an array, improving the time to O(n).

function isCyclic(object) {
   const seenObjects = new WeakMap(); // use to keep track of which objects have been seen.

   function detectCycle(obj) {
      // If 'obj' is an actual object (i.e., has the form of '{}'), check
      // if it's been seen already.
      if (Object.prototype.toString.call(obj) == '[object Object]') {

         if (seenObjects.has(obj)) {
            return true;
         }

         // If 'obj' hasn't been seen, add it to 'seenObjects'.
         // Since 'obj' is used as a key, the value of 'seenObjects[obj]'
         // is irrelevent and can be set as literally anything you want. I 
         // just went with 'undefined'.
         seenObjects.set(obj, undefined);

         // Recurse through the object, looking for more circular references.
         for (var key in obj) {
            if (detectCycle(obj[key])) {
               return true;
            }
         }

      // If 'obj' is an array, check if any of it's elements are
      // an object that has been seen already.
      } else if (Array.isArray(obj)) {
         for (var i in obj) {
            if (detectCycle(obj[i])) {
               return true;
            }
         }
      }

      return false;
   }

   return detectCycle(object);
}

And this is what it looks like in action.

> var foo = {grault: {}};
> detectCycle(foo);
false
> foo.grault = foo;
> detectCycle(foo);
true
> var bar = {};
> detectCycle(bar);
false
> bar.plugh = [];
> bar.plugh.push(bar);
> detectCycle(bar);
true