SponsorBlock

cytube-sponsorblock.js is not part of the extension. It is a self-contained IIFE you paste into a CyTube channel’s JS field, where it runs in the browser of every viewer who has channel JS enabled.

What it does

1. Detects the currently playing media and, for YouTube media only, fetches its SponsorBlock segments.

2. Merges segments that nearly touch into single skip ranges.

3. Polls playback position and seeks past a range whenever playback enters one.

4. Keeps a small status pill on the page reporting its state.

5. Propagates the skip when the local user is the channel leader, and compensates locally when there is no leader.

It guards against double installation with a window.__cytubeSponsorBlockInstalled flag, so pasting it twice or re-running it is harmless.

Media detection

Two sources are combined. window.PLAYER.mediaType and PLAYER.mediaId give the live state, and CyTube’s Callbacks.changeMedia is wrapped to record a “hint” of the media that is about to load.

The hint wins for up to 10 seconds while PLAYER is still catching up, and is cleared as soon as PLAYER agrees with it. Media is keyed as type:id, and any key change resets all per-video state.

Non-YouTube media is recognised and ignored - the pill reads SponsorBlock idle: not a YouTube video.

Segment fetching and merging

Segments are requested from the SponsorBlock API with service=YouTube and actionType=skip, one category= parameter per configured category. Responses are normalised (entries without a valid two-element segment array are dropped, each keeps its category), sorted by start time, then merged: two ranges closer than mergeGapSeconds (0.35 s by default) become one.

Merging matters because seeking is not free. Two sponsor segments a fifth of a second apart would otherwise cause two visible seeks in a row.

A response that arrives after the media changed is discarded rather than applied to the wrong video.

Skipping

Every pollMs (400 ms by default) the script reads PLAYER.getTime(), finds any merged range containing the current position, and seeks to range.end + seekPaddingSeconds.

Guards prevent seek loops:

• The player must be ready for the media the script currently tracks.
• Re-skipping the same range within 1.5 s is suppressed when playback is already at or past the previous target.
• A target that is not actually ahead of the current position is ignored.

Seeking probes several APIs in order, because the method available depends on which player CyTube instantiated: PLAYER.seekTo, PLAYER.seek, PLAYER.setTime, then PLAYER.player.seekTo, PLAYER.player.seek, PLAYER.player.setCurrentTime, and finally assigning PLAYER.player.currentTime. If none exist, the pill reports SponsorBlock could not seek this player.

Staying in sync

This is the awkward part of doing SponsorBlock in a synchronised watch-party, and the script handles two cases differently.

With a leader

If CLIENT.leader is true, the script emits a mediaUpdate shortly after seeking:

socket.emit("mediaUpdate", {
  id: PLAYER.mediaId,
  currentTime: <seconds>,
  paused: Boolean(PLAYER.paused),
  type: PLAYER.mediaType
});

That is CyTube’s normal sync path, so the whole room follows the skip.

With no leader (server autolead)

Without a leader, the server keeps its own clock, which knows nothing about your local skips. The script therefore accumulates a localSyncOffset - the total time it has skipped - and adds it to the currentTime of incoming changeMedia and mediaUpdate packets before CyTube’s own handlers see them. Each range contributes to the offset only once.

The offset resets on media change and whenever Callbacks.setLeader fires, since the sync regime just changed.

Viewers can drift apart

In a no-leader room, viewers running channel JS skip locally and end up ahead of viewers who have channel JS disabled. This is inherent to a client-side approach and is not something the script can fix.

Status pill

With ui: true the script renders a small pill on the page, tinted by tone, showing messages such as:

• SponsorBlock waiting for media
• SponsorBlock loading segments...
• SponsorBlock ready: 3 skip range(s)
• SponsorBlock active: no segments for this video
• SponsorBlock skipped to 4:21
• SponsorBlock request failed (500)

A 1-second interval keeps it current. Set ui: false for a silent install, and debug: true for [CyTube SponsorBlock] console logging.

Layout tweak

Before installing the skip logic the script also widens the video pane by repeatedly calling a re-implemented CyTube.ui.changeVideoWidth, which clamps between Bootstrap columns 3 and 10. This is a cosmetic convenience unrelated to skipping; it is wrapped in a try/catch and logs a warning if the layout does not support it (for example the hd layout). Delete applyExistingLayoutTweaks and its call if you do not want it.

Limits

• It is channel JS, not a server modification. Anyone with channel JS disabled gets none of it.
• Skips are computed and executed per viewer, locally.
• Room-wide sync updates are only sent by the current leader.
• Non-YouTube media is ignored entirely.
• Segment data quality is whatever SponsorBlock’s contributors submitted for that video.