fix circular objects in json reporter. Closes #2433

lib/reporters/json.js

@@ -67,17 +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

mocha.js

@@ -3074,17 +3074,43 @@ 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

test/reporters/json.spec.js

@@ -60,4 +60,31 @@ describe('json reporter', function () {
       done();
     });
   });
+
+  it('should handle circular objects in errors', function (done) {
+    var testTitle = 'json test 1';
+    function CircleError () {
+      this.message = 'oh shit';
+      this.circular = this;
+    }
+    var error = new CircleError();
+
+    suite.addTest(new Test(testTitle, function (done) {
+      throw error;
+    }));
+
+    runner.run(function (failureCount) {
+      failureCount.should.be.exactly(1);
+      runner.should.have.property('testResults');
+      runner.testResults.should.have.property('failures');
+      runner.testResults.failures.should.be.an.instanceOf(Array);
+      runner.testResults.failures.should.have.a.lengthOf(1);
+
+      var failure = runner.testResults.failures[0];
+      failure.should.have.property('title', testTitle);
+      failure.should.have.properties('err');
+
+      done();
+    });
+  });
 });