Limit inProgress using maxItems

lib/index.js

@@ -211,7 +211,9 @@ Queue.prototype._processHead = function() {
     });
   }
 
-  while (queue.length && queue[0].time <= now) {
+  var inProgressSize = Object.keys(inProgress).length;
+
+  while (queue.length && queue[0].time <= now && inProgressSize++ < self.maxItems) {
     var el = queue.shift();
     var id = uuid();
 

test/index.test.js

@@ -237,6 +237,63 @@ describe('Queue', function() {
       assert(call === queue.maxAttempts + 1);
     });
   });
+
+  it('should limit inProgress using maxItems', function() {
+    var waiting = [];
+    var i;
+
+    queue.maxItems = 100;
+    queue.maxAttempts = 2;
+    queue.fn = function(_, done) {
+      waiting.push(done);
+    };
+
+    // add maxItems * 2 items
+    for (i = 0; i < queue.maxItems * 2; i++) {
+      queue.addItem({ index: i });
+    }
+
+    // the queue should be full
+    assert(size(queue).queue === queue.maxItems);
+
+    queue.start();
+    // the queue is now empty and everything is in progress
+    assert(size(queue).queue === 0);
+    assert(size(queue).inProgress === queue.maxItems);
+
+    // while the items are in progress let's add maxItems times two items
+    for (i = 0; i < queue.maxItems * 2; i++) {
+      queue.addItem({ index: i });
+    }
+
+    // inProgress and queue should be full
+    assert(size(queue).queue === queue.maxItems);
+    assert(size(queue).inProgress === queue.maxItems);
+    assert(waiting.length === queue.maxItems);
+
+    // resolved all waiting items
+    while (waiting.length) {
+      waiting.pop()();
+    }
+
+    // inProgress should now be empty
+    assert(size(queue).queue === queue.maxItems);
+    assert(size(queue).inProgress === 0);
+
+    // wait for the queue to be processed
+    clock.tick(queue.getDelay(0));
+
+    // items should now be in progress
+    assert(size(queue).queue === 0);
+    assert(size(queue).inProgress === queue.maxItems);
+
+    function size(queue) {
+      return {
+        queue: queue._store.get(queue.keys.QUEUE).length,
+        inProgress: Object.keys(queue._store.get(queue.keys.IN_PROGRESS) || {}).length
+      };
+    }
+  });
 });
 
 describe('events', function() {