[Fix] handle circular references

 - See nodejs/node@d3aafd0
 - Fixes #19
 - Will allow tape-testing/tape#434 to be fixed

.eslintrc

@@ -6,8 +6,9 @@
     "eqeqeq": 1,
     "func-style": [2, "declaration"],
     "indent": [2, 2],
-    "max-params": [2, 3],
+    "max-params": [2, 4],
     "max-statements-per-line": [2, { "max": 2 }],
+    "operator-linebreak": [2, "before"],
     "strict": 1,
   },
   "globals": {

index.js

@@ -18,7 +18,7 @@ var $mapHas = callBound('Map.prototype.has', true);
 var $mapGet = callBound('Map.prototype.get', true);
 var $setHas = callBound('Set.prototype.has', true);
 
-function deepEqual(actual, expected, options) {
+function internalDeepEqual(actual, expected, options, memos) {
   var opts = options || {};
 
   // 7.1. All identical values are equivalent, as determined by ===.
@@ -46,8 +46,20 @@ function deepEqual(actual, expected, options) {
    * corresponding key, and an identical 'prototype' property. Note: this
    * accounts for both named and indexed properties on Arrays.
    */
+  // see https://github.com/nodejs/node/commit/d3aafd02efd3a403d646a3044adcf14e63a88d32 for memos inspiration
+
+  var actualIndex = memos.actual.indexOf(actual);
+  if (actualIndex !== -1) {
+    if (actualIndex === memos.expected.indexOf(expected)) {
+      return true;
+    }
+  }
+
+  memos.actual.push(actual);
+  memos.expected.push(expected);
+
   // eslint-disable-next-line no-use-before-define
-  return objEquiv(actual, expected, opts);
+  return objEquiv(actual, expected, opts, memos);
 }
 
 function isUndefinedOrNull(value) {
@@ -67,7 +79,7 @@ function isBuffer(x) {
   return true;
 }
 
-function objEquiv(a, b, opts) {
+function objEquiv(a, b, opts, memos) {
   /* eslint max-statements: [2, 100], max-lines-per-function: [2, 120], max-depth: [2, 5] */
   var i, key;
 
@@ -141,7 +153,7 @@ function objEquiv(a, b, opts) {
   // equivalent values for every corresponding key, and ~~~possibly expensive deep test
   for (i = ka.length - 1; i >= 0; i--) {
     key = ka[i];
-    if (!deepEqual(a[key], b[key], opts)) { return false; }
+    if (!internalDeepEqual(a[key], b[key], opts, memos)) { return false; }
   }
 
   var aCollection = whichCollection(a);
@@ -160,11 +172,14 @@ function objEquiv(a, b, opts) {
       while ((resultA = iA.next()) && (resultB = iB.next()) && !resultA.done && !resultB.done) {
         if (!$mapHas(a, resultB.value[0]) || !$mapHas(b, resultA.value[0])) { return false; }
         if (resultA.value[0] === resultB.value[0]) { // optimization: keys are the same, no need to look up values
-          if (!deepEqual(resultA.value[1], resultB.value[1], opts)) { return false; }
+          if (!internalDeepEqual(resultA.value[1], resultB.value[1], opts, memos)) { return false; }
         } else {
           aWithBKey = $mapGet(a, resultB.value[0]);
           bWithAKey = $mapGet(b, resultA.value[0]);
-          if (!deepEqual(resultA.value[1], bWithAKey, opts) || !deepEqual(resultB.value[1], aWithBKey, opts)) {
+          if (
+            !internalDeepEqual(resultA.value[1], bWithAKey, opts, memos)
+            || !internalDeepEqual(resultB.value[1], aWithBKey, opts, memos)
+          ) {
             return false;
           }
         }
@@ -182,4 +197,6 @@ function objEquiv(a, b, opts) {
   return true;
 }
 
-module.exports = deepEqual;
+module.exports = function deepEqual(a, b, opts) {
+  return internalDeepEqual(a, b, opts, { actual: [], expected: [] });
+};

test/cmp.js

@@ -625,3 +625,37 @@ test('fake arrays: extra keys will be tested', { skip: [].__proto__ !== Array.pr
   t.deepEqualTest(a, [1, 1], 'fake and real array with same contents and [[Prototype]]', false, false);
   t.end();
 });
+
+test('circular references', function (t) {
+  var b = {};
+  b.b = b;
+
+  var c = {};
+  c.b = c;
+
+  t.deepEqualTest(
+    b,
+    c,
+    'two self-referencing objects',
+    true,
+    true
+  );
+
+  var d = {};
+  d.a = 1;
+  d.b = d;
+
+  var e = {};
+  e.a = 1;
+  e.b = e.a;
+
+  t.deepEqualTest(
+    d,
+    e,
+    'two deeply self-referencing objects',
+    false,
+    false
+  );
+
+  t.end();
+});