Issue #329, special check for the common and magical 'length' property.

test/objects.js

@@ -183,7 +183,7 @@ $(document).ready(function() {
 
     // Sparse arrays.
     ok(_.isEqual(Array(3), Array(3)), "Sparse arrays of identical lengths are equal");
-    ok(_.isEqual(Array(3), Array(6)), "Sparse arrays of different lengths are equal when both are empty");
+    ok(!_.isEqual(Array(3), Array(6)), "Sparse arrays of different lengths are not equal when both are empty");
 
     // According to the Microsoft deviations spec, section 2.1.26, JScript 5.x treats `undefined`
     // elements in arrays as elisions. Thus, sparse arrays and dense arrays containing `undefined`

underscore.js

@@ -675,6 +675,8 @@
     }
     // Ensure that both values are objects.
     if (typeA != 'object') return false;
+    // Arrays or Arraylikes with different lengths are not equal.
+    if (a.length !== b.length) return false;
     // Objects with different constructors are not equal.
     if (a.constructor !== b.constructor) return false;
     // Assume equality for cyclic structures. The algorithm for detecting cyclic structures is