hakimel/reveal.js · error · Error

Please specify a valid media type to preview

Error message

Please specify a valid media type to preview

What it means

Overlay.previewMedia(url, mediaType, fitMode) only recognizes two media types: 'image' and 'video' (branches at overlay.js:135 and :162). Any other mediaType value falls through to the trailing else and throws. The throw happens before any DOM or network work, so it is a pure argument-validation failure, not a load failure.

Source

Thrown at js/controllers/overlay.js:189

			video.playsInline = true;
			video.src = url;
			contentElement.appendChild( video );

			video.addEventListener( 'loadeddata', () => {
				this.dom.dataset.state = 'loaded';
			}, false );

			video.addEventListener( 'error', () => {
				this.dom.dataset.state = 'error';
				contentElement.innerHTML =
					`<span class="r-overlay-error">Unable to load video.</span>`;
			}, false );

			this.Reveal.dispatchEvent({ type: 'previewvideo', data: { url } });

		}
		else {
			throw new Error( 'Please specify a valid media type to preview' );
		}

		this.dom.querySelector( '.r-overlay-close' ).addEventListener( 'click', ( event ) => {
			this.close();
			event.preventDefault();
		}, false );

	}

	previewImage( url, fitMode ) {

		this.previewMedia( url, 'image', fitMode );

	}

	previewVideo( url, fitMode ) {

		this.previewMedia( url, 'video', fitMode );

View on GitHub (pinned to a3b9406956)

Solutions

1. Pass one of the two supported literals: 'image' or 'video'. Prefer the previewImage(url, fitMode) / previewVideo(url, fitMode) helpers (overlay.js:199) so the type is hardcoded.
2. If mediaType comes from dynamic input, normalize it first: map MIME prefixes (image/* -> 'image', video/* -> 'video') and reject everything else before calling previewMedia.
3. Guard the call: only invoke previewMedia when the type is in {'image','video'}; otherwise log and skip rather than letting it throw.
4. Do not pass undefined hoping for a default — there is no default branch, only the throw.

Example fix

// before
overlay.previewMedia(url, fileType, fit); // fileType may be 'audio' or undefined

// after
if (fileType === 'image' || fileType === 'video') {
  overlay.previewMedia(url, fileType, fit);
} else {
  console.warn(`Unsupported preview media type: ${fileType}`);
}

const VALID_MEDIA_TYPES = new Set(['image', 'video']);
function safePreviewMedia(overlay, url, mediaType, fitMode) {
  if (!VALID_MEDIA_TYPES.has(mediaType)) {
    console.warn(`previewMedia: unsupported mediaType '${mediaType}'; skipping.`);
    return;
  }
  overlay.previewMedia(url, mediaType, fitMode);
}

function isOverlayMediaType(value) {
  return value === 'image' || value === 'video';
}

When it happens

Trigger: Calling previewMedia (or a wrapper) with mediaType set to anything other than the literal strings 'image' or 'video' — e.g. 'audio', 'pdf', undefined, null, a typo like 'images', or a value read from an untrusted source. Also triggered when previewImage/previewVideo helpers are bypassed and a raw type string is passed in.

Common situations: Deriving mediaType from a file extension or MIME type and forgetting to normalize to the two accepted strings; passing an optional config field that was never set (undefined); copy-pasting a call without the type argument; attempting to preview a media kind reveal.js overlay does not support (audio, iframe, pdf).

Related errors

• Fragment with asChild expects exactly one React element chil
• Fragment with asChild expects exactly one non-Fragment React

AI-assisted analysis of hakimel/reveal.js@a3b9406956 (2026-08-12). Data as JSON: /api/errors/8eef8235360d538a. Report an issue: GitHub.