Ensure _.debounce performs a trailing call, when both leading and trailing are true, only after the throttled function is called more than once. [closes #243]

Former-commit-id: 59ac6bc4ece373623f2bd753b662b8cc974cabc8

lodash.js

@@ -4478,13 +4478,14 @@
      */
     function debounce(func, wait, options) {
       var args,
+          inited,
           result,
           thisArg,
           timeoutId,
           trailing = true;
 
       function delayed() {
-        timeoutId = null;
+        inited = timeoutId = null;
         if (trailing) {
           result = func.apply(thisArg, args);
         }
@@ -4497,15 +4498,15 @@
         trailing = 'trailing' in options ? options.trailing : trailing;
       }
       return function() {
-        var isLeading = leading && !timeoutId;
         args = arguments;
         thisArg = this;
-
         clearTimeout(timeoutId);
-        timeoutId = setTimeout(delayed, wait);
 
-        if (isLeading) {
+        if (!inited && leading) {
+          inited = true;
           result = func.apply(thisArg, args);
+        } else {
+          timeoutId = setTimeout(delayed, wait);
         }
         return result;
       };
@@ -4706,10 +4707,9 @@
           trailing = true;
 
       function trailingCall() {
-        lastCalled = new Date;
         timeoutId = null;
-
         if (trailing) {
+          lastCalled = new Date;
           result = func.apply(thisArg, args);
         }
       }

test/test.js

@@ -552,16 +552,29 @@
       }, 64);
     });
 
-    test('should work with `leading` option', function() {
-      _.each([true, { 'leading': true }], function(options) {
-        var withLeading = _.debounce(_.identity, 32, options);
+    asyncTest('should work with `leading` option', function() {
+      var counts = [0, 0, 0];
+      _.each([true, { 'leading': true }], function(options, index) {
+        var withLeading = _.debounce(function(value) {
+          counts[index]++;
+          return value;
+        }, 32, options);
+
         equal(withLeading('x'), 'x');
       });
 
+      _.times(2, _.debounce(function() { counts[2]++; }, 32, { 'leading': true }));
+      strictEqual(counts[2], 1);
+
       _.each([false, { 'leading': false }], function(options) {
         var withoutLeading = _.debounce(_.identity, 32, options);
         strictEqual(withoutLeading('x'), undefined);
       });
+
+      setTimeout(function() {
+        deepEqual(counts, [1, 1, 2]);
+        QUnit.start();
+      }, 64);
     });
 
     asyncTest('should work with `trailing` option', function() {