feat: change error comparison algorithm (#57)

* chore: drop support for Node versions older than 6

BREAKING CHANGE: This commit drops support for versions of Node
that are no longer maintained.

* feat: change error comparison algorithm

BREAKING CHANGE: Previously, `Error` objects were compared using
strict equality. This commit causes `Error` objects to be compared
using deep equality instead, but treats them as a special case by
including their `name` and `message` properties in the comparison,
regardless of enumerability.

.travis.yml

@@ -6,8 +6,9 @@ cache:
   directories:
   - node_modules
 node_js:
- - 4 # to be removed 2018-04-01
- - 6 # to be removed 2019-04-01
+ - 6 # to be removed 2019-04-30
+ - 8 # to be removed 2019-12-31
+ - 10 # to be removed 2021-04-30
  - lts/* # safety net; don't remove
  - node # safety net; don't remove
 before_install:

README.md

@@ -108,9 +108,11 @@ The primary export of `deep-eql` is function that can be given two objects to co
 - All own and inherited enumerable properties are considered:
   - `eql(Object.create({ foo: { a: 1 } }), Object.create({ foo: { a: 1 } })).should.be.true;`
   - `eql(Object.create({ foo: { a: 1 } }), Object.create({ foo: { a: 2 } })).should.be.false;`
+- When comparing `Error` objects, `name` and `message` properties are also considered, regardless of enumerability:
+  - `eql(Error('foo'), Error('foo')).should.be.true;`
+  - `eql(Error('foo'), Error('bar')).should.be.false;`
+  - `eql(Error('foo'), TypeError('foo')).should.be.false;`
+  - `eql(Object.assign(Error('foo'), { code: 42 }), Object.assign(Error('foo'), { code: 13 })).should.be.false;`
 - Arguments are not Arrays:
   - `eql([], arguments).should.be.false;`
   - `eql([], Array.prototype.slice.call(arguments)).should.be.true;`
-- Error objects are compared by reference (see https://github.com/chaijs/chai/issues/608):
-  - `eql(new Error('msg'), new Error('msg')).should.be.false;`
-  - `var err = new Error('msg'); eql(err, err).should.be.true;`

index.js

@@ -207,7 +207,6 @@ function extensiveDeepEqualByType(leftHandOperand, rightHandOperand, leftHandTyp
     case 'function':
     case 'WeakMap':
     case 'WeakSet':
-    case 'Error':
       return leftHandOperand === rightHandOperand;
     case 'Arguments':
     case 'Int8Array':
@@ -234,7 +233,7 @@ function extensiveDeepEqualByType(leftHandOperand, rightHandOperand, leftHandTyp
     case 'Map':
       return entriesEqual(leftHandOperand, rightHandOperand, options);
     default:
-      return objectEqual(leftHandOperand, rightHandOperand, options);
+      return objectEqual(leftHandOperand, rightHandOperand, leftHandType, options);
   }
 }
 
@@ -378,6 +377,21 @@ function getEnumerableKeys(target) {
   return keys;
 }
 
+/*!
+ * Adds `Error`-specific keys to an array of object keys. We want to include these keys when comparing `Error` objects
+ * despite these keys being non-enumerable by default (and thus not added by `getEnumerableKeys`).
+ *
+ * @param {Array} keys An array of keys
+ */
+function addExtraErrorKeys(keys) {
+  if (keys.indexOf('name') === -1) { // eslint-disable-line no-magic-numbers
+    keys.push('name');
+  }
+  if (keys.indexOf('message') === -1) { // eslint-disable-line no-magic-numbers
+    keys.push('message');
+  }
+}
+
 /*!
  * Determines if two objects have matching values, given a set of keys. Defers to deepEqual for the equality check of
  * each key. If any value of the given key is not equal, the function will return false (early).
@@ -403,17 +417,22 @@ function keysEqual(leftHandOperand, rightHandOperand, keys, options) {
 
 /*!
  * Recursively check the equality of two Objects. Once basic sameness has been established it will defer to `deepEqual`
- * for each enumerable key in the object.
+ * for each enumerable key in the object. For `Error` objects, also compare `name` and `message` keys, regardless of
+ * enumerability.
  *
  * @param {Mixed} leftHandOperand
  * @param {Mixed} rightHandOperand
+ * @param {String} type of leftHandOperand
  * @param {Object} [options] (Optional)
  * @return {Boolean} result
  */
-
-function objectEqual(leftHandOperand, rightHandOperand, options) {
+function objectEqual(leftHandOperand, rightHandOperand, leftHandType, options) {
   var leftHandKeys = getEnumerableKeys(leftHandOperand);
   var rightHandKeys = getEnumerableKeys(rightHandOperand);
+  if (leftHandType === 'Error') {
+    addExtraErrorKeys(leftHandKeys);
+    addExtraErrorKeys(rightHandKeys);
+  }
   if (leftHandKeys.length && leftHandKeys.length === rightHandKeys.length) {
     leftHandKeys.sort();
     rightHandKeys.sort();

package.json

@@ -84,6 +84,6 @@
     "watchify": "^3.7.0"
   },
   "engines": {
-    "node": ">=0.12"
+    "node": ">=6"
   }
 }

test/index.js

@@ -379,13 +379,57 @@ describe('Generic', function () {
   describe('errors', function () {
 
     it('returns true for same errors', function () {
-      var error = new Error('foo');
+      var error = Error('foo');
       assert(eql(error, error), 'eql(error, error)');
     });
 
-    it('returns false for different errors', function () {
-      assert(eql(new Error('foo'), new Error('foo')) === false,
-        'eql(new Error("foo"), new Error("foo")) === false');
+    it('returns true for errors with same name and message', function () {
+      assert(eql(Error('foo'), Error('foo')),
+        'eql(Error("foo"), Error("foo"))');
+    });
+
+    it('returns true for errors with same name and message despite different constructors', function () {
+      var err1 = Error('foo');
+      var err2 = TypeError('foo');
+      err2.name = 'Error';
+      assert(eql(err1, err2),
+        'eql(Error("foo"), Object.assign(TypeError("foo"), { name: "Error" }))');
+    });
+
+    it('returns false for errors with same name but different messages', function () {
+      assert(eql(Error('foo'), Error('bar')) === false,
+        'eql(Error("foo"), Error("bar")) === false');
+    });
+
+    it('returns false for errors with same message but different names', function () {
+      assert(eql(Error('foo'), TypeError('foo')) === false,
+        'eql(Error("foo"), TypeError("foo")) === false');
+    });
+
+    it('returns false for errors with same message but different names despite same constructors', function () {
+      var err1 = Error('foo');
+      var err2 = Error('foo');
+      err2.name = 'TypeError';
+      assert(eql(err1, err2) === false,
+        'eql(Error("foo"), Object.assign(Error("foo"), { name: "TypeError" })) === false');
+    });
+
+    it('returns true for errors with same custom property', function () {
+      var err1 = Error('foo');
+      var err2 = Error('foo');
+      err1.code = 42;
+      err2.code = 42;
+      assert(eql(err1, err2),
+        'eql(Object.assign(Error("foo"), { code: 42 }), Object.assign(Error("foo"), { code: 42 }))');
+    });
+
+    it('returns false for errors with different custom property', function () {
+      var err1 = Error('foo');
+      var err2 = Error('foo');
+      err1.code = 42;
+      err2.code = 13;
+      assert(eql(err1, err2) === false,
+        'eql(Object.assign(new Error("foo"), { code: 42 }), Object.assign(new Error("foo"), { code: 13 })) === false');
     });
 
   });