function slide()

in js/reveal.js [1259:1453]
153 lines of code
49 McCabe index (conditional complexity)

	function slide( h, v, f, origin ) {

		// Dispatch an event before the slide
		const slidechange = dispatchEvent({
			type: 'beforeslidechange',
			data: {
				indexh: h === undefined ? indexh : h,
				indexv: v === undefined ? indexv : v,
				origin
			}
		});

		// Abort if this slide change was prevented by an event listener
		if( slidechange.defaultPrevented ) return;

		// Remember where we were at before
		previousSlide = currentSlide;

		// Query all horizontal slides in the deck
		const horizontalSlides = dom.wrapper.querySelectorAll( HORIZONTAL_SLIDES_SELECTOR );

		// Abort if there are no slides
		if( horizontalSlides.length === 0 ) return;

		// If no vertical index is specified and the upcoming slide is a
		// stack, resume at its previous vertical index
		if( v === undefined && !overview.isActive() ) {
			v = getPreviousVerticalIndex( horizontalSlides[ h ] );
		}

		// If we were on a vertical stack, remember what vertical index
		// it was on so we can resume at the same position when returning
		if( previousSlide && previousSlide.parentNode && previousSlide.parentNode.classList.contains( 'stack' ) ) {
			setPreviousVerticalIndex( previousSlide.parentNode, indexv );
		}

		// Remember the state before this slide
		const stateBefore = state.concat();

		// Reset the state array
		state.length = 0;

		let indexhBefore = indexh || 0,
			indexvBefore = indexv || 0;

		// Activate and transition to the new slide
		indexh = updateSlides( HORIZONTAL_SLIDES_SELECTOR, h === undefined ? indexh : h );
		indexv = updateSlides( VERTICAL_SLIDES_SELECTOR, v === undefined ? indexv : v );

		// Dispatch an event if the slide changed
		let slideChanged = ( indexh !== indexhBefore || indexv !== indexvBefore );

		// Ensure that the previous slide is never the same as the current
		if( !slideChanged ) previousSlide = null;

		// Find the current horizontal slide and any possible vertical slides
		// within it
		let currentHorizontalSlide = horizontalSlides[ indexh ],
			currentVerticalSlides = currentHorizontalSlide.querySelectorAll( 'section' );

		// Store references to the previous and current slides
		currentSlide = currentVerticalSlides[ indexv ] || currentHorizontalSlide;

		let autoAnimateTransition = false;

		// Detect if we're moving between two auto-animated slides
		if( slideChanged && previousSlide && currentSlide && !overview.isActive() ) {

			// If this is an auto-animated transition, we disable the
			// regular slide transition
			//
			// Note 20-03-2020:
			// This needs to happen before we update slide visibility,
			// otherwise transitions will still run in Safari.
			if( previousSlide.hasAttribute( 'data-auto-animate' ) && currentSlide.hasAttribute( 'data-auto-animate' )
					&& previousSlide.getAttribute( 'data-auto-animate-id' ) === currentSlide.getAttribute( 'data-auto-animate-id' )
					&& !( ( indexh > indexhBefore || indexv > indexvBefore ) ? currentSlide : previousSlide ).hasAttribute( 'data-auto-animate-restart' ) ) {

				autoAnimateTransition = true;
				dom.slides.classList.add( 'disable-slide-transitions' );
			}

			transition = 'running';

		}

		// Update the visibility of slides now that the indices have changed
		updateSlidesVisibility();

		layout();

		// Update the overview if it's currently active
		if( overview.isActive() ) {
			overview.update();
		}

		// Show fragment, if specified
		if( typeof f !== 'undefined' ) {
			fragments.goto( f );
		}

		// Solves an edge case where the previous slide maintains the
		// 'present' class when navigating between adjacent vertical
		// stacks
		if( previousSlide && previousSlide !== currentSlide ) {
			previousSlide.classList.remove( 'present' );
			previousSlide.setAttribute( 'aria-hidden', 'true' );

			// Reset all slides upon navigate to home
			if( isFirstSlide() ) {
				// Launch async task
				setTimeout( () => {
					getVerticalStacks().forEach( slide => {
						setPreviousVerticalIndex( slide, 0 );
					} );
				}, 0 );
			}
		}

		// Apply the new state
		stateLoop: for( let i = 0, len = state.length; i < len; i++ ) {
			// Check if this state existed on the previous slide. If it
			// did, we will avoid adding it repeatedly
			for( let j = 0; j < stateBefore.length; j++ ) {
				if( stateBefore[j] === state[i] ) {
					stateBefore.splice( j, 1 );
					continue stateLoop;
				}
			}

			dom.viewport.classList.add( state[i] );

			// Dispatch custom event matching the state's name
			dispatchEvent({ type: state[i] });
		}

		// Clean up the remains of the previous state
		while( stateBefore.length ) {
			dom.viewport.classList.remove( stateBefore.pop() );
		}

		if( slideChanged ) {
			dispatchEvent({
				type: 'slidechanged',
				data: {
					indexh,
					indexv,
					previousSlide,
					currentSlide,
					origin
				}
			});
		}

		// Handle embedded content
		if( slideChanged || !previousSlide ) {
			slideContent.stopEmbeddedContent( previousSlide );
			slideContent.startEmbeddedContent( currentSlide );
		}

		// Announce the current slide contents to screen readers
		// Use animation frame to prevent getComputedStyle in getStatusText
		// from triggering layout mid-frame
		requestAnimationFrame( () => {
			announceStatus( getStatusText( currentSlide ) );
		});

		progress.update();
		controls.update();
		notes.update();
		backgrounds.update();
		backgrounds.updateParallax();
		slideNumber.update();
		fragments.update();

		// Update the URL hash
		location.writeURL();

		cueAutoSlide();

		// Auto-animation
		if( autoAnimateTransition ) {

			setTimeout( () => {
				dom.slides.classList.remove( 'disable-slide-transitions' );
			}, 0 );

			if( config.autoAnimate ) {
				// Run the auto-animation between our slides
				autoAnimate.run( previousSlide, currentSlide );
			}

		}

	}