Refactor debounce to simplify, reduce timers, fix bugs.

lodash.js

@@ -9000,14 +9000,12 @@
      * jQuery(window).on('popstate', debounced.cancel);
      */
     function debounce(func, wait, options) {
-      var args,
-          maxTimeoutId,
+      var lastArgs,
+          lastThis,
           result,
-          stamp,
-          thisArg,
-          timeoutId,
-          trailingCall,
-          lastCalled = 0,
+          timerId,
+          lastCallTime = 0,
+          lastInvokeTime = 0,
           leading = false,
           maxWait = false,
           trailing = true;
@@ -9022,94 +9020,104 @@
         trailing = 'trailing' in options ? !!options.trailing : trailing;
       }
 
-      function cancel() {
-        if (timeoutId) {
-          clearTimeout(timeoutId);
-        }
-        if (maxTimeoutId) {
-          clearTimeout(maxTimeoutId);
-        }
-        lastCalled = 0;
-        args = maxTimeoutId = thisArg = timeoutId = trailingCall = undefined;
+      function invoke(time) {
+        var args = lastArgs,
+            thisArg = lastThis;
+
+        lastArgs = lastThis = undefined;
+        lastInvokeTime = time;
+        result = func.apply(thisArg, args);
+        return result;
       }
 
-      function complete(isCalled, id) {
-        if (id) {
-          clearTimeout(id);
+      function leadingEdge() {
+        // Reset any `maxWait` timer.
+        lastInvokeTime = lastCallTime;
+        // Start the timer to the trailing edge.
+        timerId = setTimeout(timerExpired, wait);
+        // Invoke the leading edge.
+        return leading ? invoke(lastCallTime) : result;
+      }
+
+      function trailingEdge(time) {
+        if (timerId !== undefined) {
+          clearTimeout(timerId);
+          timerId = undefined;
         }
-        maxTimeoutId = timeoutId = trailingCall = undefined;
-        if (isCalled) {
-          lastCalled = now();
-          result = func.apply(thisArg, args);
-          if (!timeoutId && !maxTimeoutId) {
-            args = thisArg = undefined;
+        // Only invoke if we have `lastArgs`, which means there has been a call
+        // to `func` since the last invocation
+        if (trailing && lastArgs) {
+          return invoke(time);
+        }
+        lastArgs = lastThis = undefined;
+        return result;
+      }
+
+      function checkTimes(time) {
+        var timeSinceLastInvoke = time - lastInvokeTime,
+            waitTime = time - lastCallTime;
+
+        if (waitTime >= wait) {
+          // Activity has stopped. We are at the trailing edge.
+          trailingEdge(time);
+          return;
+        }
+        if (waitTime < 0) {
+          // The system time has gone backwards. Treat it as the trailing edge.
+          trailingEdge(time);
+          return;
+        }
+        var shouldInvoke = (maxWait !== false && timeSinceLastInvoke >= maxWait);
+
+        // Restart the timer to the smaller of remaining maxWait and remaining wait.
+        var remainingWait = wait - waitTime;
+        if (maxWait !== false) {
+          remainingWait = nativeMin(remainingWait, maxWait - timeSinceLastInvoke);
+        }
+        return {
+          'shouldInvoke': shouldInvoke,
+          'remainingWait': remainingWait
+        };
+      }
+
+      function timerExpired() {
+        var time = now(),
+            check = checkTimes(time);
+
+        timerId = undefined;
+        if (check !== undefined) {
+          // Restart the timer.
+          timerId = setTimeout(timerExpired, check.remainingWait);
+          if (check.shouldInvoke) {
+            invoke(time);
           }
         }
       }
 
-      function delayed() {
-        var remaining = wait - (now() - stamp);
-        if (remaining <= 0 || remaining > wait) {
-          complete(trailingCall, maxTimeoutId);
-        } else {
-          timeoutId = setTimeout(delayed, remaining);
+      function cancel() {
+        if (timerId !== undefined) {
+          clearTimeout(timerId);
         }
+        lastArgs = lastThis = timerId = undefined;
       }
 
       function flush() {
-        if ((timeoutId && trailingCall) || (maxTimeoutId && trailing)) {
-          result = func.apply(thisArg, args);
-        }
-        cancel();
-        return result;
-      }
-
-      function maxDelayed() {
-        complete(trailing, timeoutId);
+        return timerId === undefined ? result : trailingEdge(now());
       }
 
       function debounced() {
-        args = arguments;
-        stamp = now();
-        thisArg = this;
-        trailingCall = trailing && (timeoutId || !leading);
+        lastArgs = arguments;
+        lastThis = this;
+        lastCallTime = now();
 
-        if (maxWait === false) {
-          var leadingCall = leading && !timeoutId;
-        } else {
-          if (!lastCalled && !maxTimeoutId && !leading) {
-            lastCalled = stamp;
-          }
-          var remaining = maxWait - (stamp - lastCalled);
-
-          var isCalled = (remaining <= 0 || remaining > maxWait) &&
-            (leading || maxTimeoutId);
-
-          if (isCalled) {
-            if (maxTimeoutId) {
-              maxTimeoutId = clearTimeout(maxTimeoutId);
-            }
-            lastCalled = stamp;
-            result = func.apply(thisArg, args);
-          }
-          else if (!maxTimeoutId) {
-            maxTimeoutId = setTimeout(maxDelayed, remaining);
-          }
+        if (timerId === undefined) {
+          return leadingEdge();
         }
-        if (isCalled && timeoutId) {
-          timeoutId = clearTimeout(timeoutId);
-        }
-        else if (!timeoutId && wait !== maxWait) {
-          timeoutId = setTimeout(delayed, wait);
-        }
-        if (leadingCall) {
-          isCalled = true;
-          result = func.apply(thisArg, args);
-        }
-        if (isCalled && !timeoutId && !maxTimeoutId) {
-          args = thisArg = undefined;
-        }
-        return result;
+        // Check the current times to handle invocations in a tight loop.
+        var check = checkTimes(lastCallTime);
+        return (check && check.shouldInvoke)
+          ? invoke(lastCallTime)
+          : result;
       }
       debounced.cancel = cancel;
       debounced.flush = flush;

test/test.js

@@ -4193,6 +4193,76 @@
       }, 192);
     });
 
+    QUnit.test('should honor leading: false when maxWait is not supplied', function(assert) {
+      assert.expect(6);
+
+      var done = assert.async();
+
+      var callCount = 0;
+
+      var debounced = _.debounce(function(value) {
+        ++callCount;
+        return value;
+      }, 32);
+
+      // Leading should not fire.
+      var actual = [debounced(0), debounced(1), debounced(2)];
+      assert.deepEqual(actual, [undefined, undefined, undefined]);
+      assert.strictEqual(callCount, 0);
+
+      setTimeout(function() {
+        // Trailing should fire by now.
+        assert.strictEqual(callCount, 1);
+
+        // Do it again.
+        var actual = [debounced(4), debounced(5), debounced(6)];
+
+        // Previous result.
+        assert.deepEqual(actual, [2, 2, 2]);
+        assert.strictEqual(callCount, 1);
+      }, 128);
+
+      setTimeout(function() {
+        assert.strictEqual(callCount, 2);
+        done();
+      }, 256);
+    });
+
+    QUnit.test('should honor leading: false when maxWait is supplied', function(assert) {
+      assert.expect(6);
+
+      var done = assert.async();
+
+      var callCount = 0;
+
+      var debounced = _.debounce(function(value) {
+        ++callCount;
+        return value;
+      }, 32, { 'maxWait': 64 });
+
+      // Leading should not fire.
+      var actual = [debounced(0), debounced(1), debounced(2)];
+      assert.deepEqual(actual, [undefined, undefined, undefined]);
+      assert.strictEqual(callCount, 0);
+
+      setTimeout(function() {
+        // Trailing should fire by now.
+        assert.strictEqual(callCount, 1);
+
+        // Do it again.
+        var actual = [debounced(4), debounced(5), debounced(6)];
+
+        // Previous result.
+        assert.deepEqual(actual, [2, 2, 2]);
+        assert.strictEqual(callCount, 1);
+      }, 128);
+
+      setTimeout(function() {
+        assert.strictEqual(callCount, 2);
+        done();
+      }, 256);
+    });
+
     QUnit.test('should invoke the `trailing` call with the correct arguments and `this` binding', function(assert) {
       assert.expect(2);
 
@@ -21271,7 +21341,7 @@
     });
 
     QUnit.test('should trigger a second throttled call as soon as possible', function(assert) {
-      assert.expect(2);
+      assert.expect(3);
 
       var done = assert.async();
 
@@ -21288,10 +21358,14 @@
         throttled();
       }, 192);
 
+      setTimeout(function() {
+        assert.strictEqual(callCount, 1);
+      }, 254);
+
       setTimeout(function() {
         assert.strictEqual(callCount, 2);
         done();
-      }, 288);
+      }, 384);
     });
 
     QUnit.test('should apply default options', function(assert) {