move circular object logic into clean

lib/reporters/json.js

@@ -54,18 +54,7 @@ function JSONReporter (runner) {
 
     runner.testResults = obj;
 
-    var cache = [];
-    process.stdout.write(JSON.stringify(obj, function (key, value) {
-      if (typeof value === 'object' && value !== null) {
-        if (cache.indexOf(value) !== -1) {
-          // Instead of going in a circle, we'll print [object Object]
-          return '' + value;
-        }
-        cache.push(value);
-      }
-
-      return value;
-    }, 2));
+    process.stdout.write(JSON.stringify(obj, null, 2));
   });
 }
 
@@ -78,17 +67,43 @@ function JSONReporter (runner) {
  * @return {Object}
  */
 function clean (test) {
+  var err = test.err || {};
+  if (err instanceof Error) {
+    err = errorJSON(err);
+  }
+
   return {
     title: test.title,
     fullTitle: test.fullTitle(),
     duration: test.duration,
     currentRetry: test.currentRetry(),
-    err: errorJSON(test.err || {})
+    err: cleanCycles(err)
   };
 }
+/**
+ * Replaces any circular references inside `obj` with '[object Object]'
+ *
+ * @api private
+ * @param {Object} obj
+ * @return {Object}
+ */
+function cleanCycles (obj) {
+  var cache = [];
+  return JSON.parse(JSON.stringify(obj, function (key, value) {
+    if (typeof value === 'object' && value !== null) {
+      if (cache.indexOf(value) !== -1) {
+        // Instead of going in a circle, we'll print [object Object]
+        return '' + value;
+      }
+      cache.push(value);
+    }
+
+    return value;
+  }));
+}
 
 /**
- * Transform `error` into a JSON object.
+ * Transform an Error object into a JSON object.
  *
  * @api private
  * @param {Error} err