Fix race

extensions/amp-carousel/0.1/slidescroll.js

@@ -521,6 +521,11 @@ export class AmpSlideScroll extends BaseSlides {
    * @return {number} a number representing the next slide index.
    */
   getNextSlideIndex_(currentScrollLeft) {
+    // Addresses race where slideWidth is 0, due to being hidden
+    // while snapping is occuring.
+    if (!currentScrollLeft && !this.slideWidth_) {
+      return 0;
+    }
     // This can be only 0, 1 or 2, since only a max of 3 slides are shown at
     // a time.
     const scrolledSlideIndex = Math.round(currentScrollLeft / this.slideWidth_);
@@ -686,10 +691,7 @@ export class AmpSlideScroll extends BaseSlides {
    */
   showSlide_(newIndex) {
     const {noOfSlides_} = this;
-    if (isNaN(newIndex)) {
-      dev().error(TAG, 'Attempted to show slide that is not a number');
-      return;
-    }
+    newIndex = dev().assertNumber(newIndex);
     if (
       newIndex < 0 ||
       newIndex >= noOfSlides_ ||

extensions/amp-carousel/0.1/test/test-slidescroll.js

@@ -22,8 +22,8 @@ import {
   createElementWithAttributes,
   whenUpgradedToCustomElement,
 } from '../../../../src/dom';
-import {dev, user} from '../../../../src/log';
 import {installResizeObserverStub} from '../../../../testing/resize-observer-stub';
+import {user} from '../../../../src/log';
 
 describes.realWin(
   'SlideScroll',
@@ -1271,25 +1271,15 @@ describes.realWin(
         expect(showSlideSpy).to.have.been.calledWith(4);
       });
 
-      it('should error on a non-number goToSlide', async () => {
+      it('should handle carousel snapping & hiding race', async () => {
         const ampSlideScroll = await getAmpSlideScroll(true);
         const impl = await ampSlideScroll.getImpl();
-        const devErrorSpy = env.sandbox.spy(dev(), 'error');
 
-        impl.showSlideAndTriggerAction_(NaN);
-        expect(devErrorSpy).to.be.calledOnce;
-        expect(devErrorSpy.args[0][1]).to.match(
-          /Attempted to show slide that is not a number/
-        );
-      });
-
-      it('should return false for invalid slide number', async () => {
-        const ampSlideScroll = await getAmpSlideScroll(true);
-        const impl = await ampSlideScroll.getImpl();
+        // simluate carousel hidding
+        impl.slideWidth_ = 0;
 
-        impl.noOfSlides_ = 10;
-        expect(impl.showSlide_(Infinity)).to.be.false;
-        expect(impl.showSlide_(-Infinity)).to.be.false;
+        // simulate snapping
+        expect(impl.getNextSlideIndex_(0)).to.equal(0);
       });
 
       it('should NOT call showSlide_ before layout', async () => {