fix(carousel): setInterval memory leak when no slides provided (#2399)

src/components/carousel/carousel.js

@@ -213,8 +213,7 @@ export default {
       attributeFilter: [ 'id' ]
     })
   },
-  /* istanbul ignore next: dificult to test */
-  beforeDestroy () {
+  beforeDestroy () /* istanbul ignore next: dificult to test */ {
     clearInterval(this.intervalId)
     clearTimeout(this._animationTimeout)
     this.intervalId = null
@@ -224,6 +223,7 @@ export default {
     // Set slide
     setSlide (slide) {
       // Don't animate when page is not visible
+      /* istanbul ignore if: dificult to test */
       if (typeof document !== 'undefined' && document.visibilityState && document.hidden) {
         return
       }
@@ -266,8 +266,8 @@ export default {
     },
     // Start auto rotate slides
     start () {
-      // Don't start if no interval, or if we are already running
-      if (!this.interval || this.isCycling) {
+      // Don't start if no interval, no slides, or if we are already running
+      if (!this.interval || this.isCycling || this.intervalId || this.slides.length === 0) {
         return
       }
       this.slides.forEach(slide => {
@@ -279,6 +279,7 @@ export default {
     },
     // Re-Start auto rotate slides when focus/hover leaves the carousel
     restart (evt) {
+      /* istanbul ignore if: dificult to test */
       if (!this.$el.contains(document.activeElement)) {
         this.start()
       }

src/components/carousel/carousel.spec.js

@@ -70,4 +70,29 @@ describe('carousel', async () => {
       expect(carousel.isSliding).toBe(false)
     })
   })
+
+  it('Should scroll to specified slide', async () => {
+    const { app } = window
+    const carousel = app.$refs.carousel
+
+    const spyBegin = jest.fn()
+    const spyEnd = jest.fn()
+
+    carousel.$on('sliding-start', spyBegin)
+    carousel.$on('sliding-end', spyEnd)
+
+    app.slide = 2
+
+    app.$nextTick(() => {
+      expect(spyBegin).toHaveBeenCalled()
+      expect(carousel.isSliding).toBe(true)
+    })
+
+    jest.runAllTimers()
+
+    app.$nextTick(() => {
+      expect(spyEnd).toHaveBeenCalledWith(app.slide)
+      expect(carousel.isSliding).toBe(false)
+    })
+  })
 })

src/components/carousel/fixtures/carousel.html

@@ -30,8 +30,16 @@ <h1>Hello world!</h1>
 
     </b-carousel>
 
-    <p class="mt-4">
+    <p class="mt-4 mb-4">
         Slide #: {{ slide }}<br>
         Is Sliding: {{ sliding }}
     </p>
+
+    <!-- empty carousel -->
+    <b-carousel id="carousel2"
+                ref="carousel2"
+                controls
+                indicators
+                background="grey">
+    </b-carousel>
 </div>