JSON.stringify() - debug TypeError: Converting circular structure to JSON

test-stringify-circular.js

'use strict';

function replacer() {
  var objects = [];

  return function(key, value) {
    if (typeof value === 'object' && value !== null) {
      var found = objects.some(function(existing) {
        return (existing === value);
      });

      if (found) {
        return '[Circular: ' + key + ']';
      }

      objects.push(value);
    }

    return value;
  };
}

var b = {};
var a = {
  x: {
    y: {
      z: b
    }
  }
};

b.a = a;

console.log(JSON.stringify(a, replacer(), '  '));