Ensure _.debounce with leading and trailing will call the func on the leading edge after a trailing call is performed. [closes #257]

Former-commit-id: 97afe842b2b4c3eb20c9557298e01ec268386ea2

lodash.js

@@ -4482,6 +4482,7 @@
      */
     function debounce(func, wait, options) {
       var args,
+          calledTwice,
           inited,
           result,
           thisArg,
@@ -4489,8 +4490,9 @@
           trailing = true;
 
       function delayed() {
-        inited = timeoutId = null;
-        if (trailing) {
+        var callTrailing = trailing && ((leading && calledTwice) || !leading);
+        calledTwice = inited = timeoutId = null;
+        if (callTrailing) {
           result = func.apply(thisArg, args);
         }
       }
@@ -4510,8 +4512,9 @@
           inited = true;
           result = func.apply(thisArg, args);
         } else {
-          timeoutId = setTimeout(delayed, wait);
+          calledTwice = true;
         }
+        timeoutId = setTimeout(delayed, wait);
         return result;
       };
     }

test/test.js

@@ -558,14 +558,19 @@
     });
 
     asyncTest('should work with `leading` option', function() {
-      var counts = [0, 0, 0];
+      var withLeadingAndTrailing,
+          counts = [0, 0, 0];
+
       _.each([true, { 'leading': true }], function(options, index) {
-        var withLeading = _.debounce(function(value) {
+        var debounced = _.debounce(function(value) {
           counts[index]++;
           return value;
         }, 32, options);
 
-        equal(withLeading('x'), 'x');
+        if (index == 1) {
+          withLeadingAndTrailing = debounced;
+        }
+        equal(debounced('x'), 'x');
       });
 
       _.times(2, _.debounce(function() { counts[2]++; }, 32, { 'leading': true }));
@@ -578,6 +583,10 @@
 
       setTimeout(function() {
         deepEqual(counts, [1, 1, 2]);
+
+        withLeadingAndTrailing('x');
+        equal(counts[1], 2);
+
         QUnit.start();
       }, 64);
     });