YouTube A/B Loop

June 21, 2026 · View on GitHub

A lightweight userscript that adds a native A/B loop panel directly into the YouTube player — no extension, no bloat. Works with ScriptCat, Tampermonkey, and any compatible userscript manager.

alt text

Features

A → B loop — set two timestamps and loop between them indefinitely
Full video loop — restart the entire video automatically on end
Manual time entry — click a point's time to type an exact timestamp (strict m:ss / h:mm:ss), clamped to the video length
Mini timeline — draggable thumbs for A, B and the playhead, with a highlighted range overlay
Keyboard shortcuts — press A or B to set points at the current time
Auto-update banner — the panel shows a notification when a newer version is available on GitHub
Native look — glass-morphism panel that matches YouTube's own UI style
Zero performance waste — RAF-based render loop with per-frame value caching; only writes to the DOM when something actually changes

Screenshots

Panel — A/B mode	Panel — Full video mode

Installation

Requirements

A userscript manager installed in your browser. Recommended options:

Manager	Browsers	Link
ScriptCat ⭐	Chrome, Firefox, Edge	scriptcat.org
Tampermonkey	Chrome, Firefox, Edge, Safari	tampermonkey.net
Violentmonkey	Chrome, Firefox, Edge	violentmonkey.github.io

One-click install

Click the link below — your userscript manager will open an install prompt automatically:

→ Install YouTube A/B Loop

Manual install

Open ScriptCat (or Tampermonkey) → Create a new script
Paste the contents of youtube-loop.js
Save (Ctrl+S)

Usage

Action	How
Open the panel	Click the ⊞ button in YouTube's right toolbar
Set point A	Click Set on the A card, or press `A`
Set point B	Click Set on the B card, or press `B`
Edit a timestamp	Click the time on the A/B card → type `m:ss` or `h:mm:ss`, `Enter` to confirm, `Esc` to cancel
Clear a point	Click ✕ next to the point
Enable loop	Toggle the Loop off / Loop on pill switch
Switch mode	Click Full video or A → B
Reset everything	Click Reset
Seek	Click anywhere on the mini-timeline, or drag any thumb

Tip: You can set A and B while the video is playing — the loop activates immediately. You can also click a point's time and type an exact value (even on an unset point), so there's no need to seek to the position first.

Auto-update

ScriptCat and Tampermonkey both check for updates automatically (once per day by default) using the @updateURL and @downloadURL directives in the script header.

When a new version is published on GitHub, a yellow "Update available" banner also appears at the bottom of the panel with a direct Install link.

To bump the version yourself, update both:

@version in the userscript header
VERSION constant in the code

How it works

tryInject()  →  inject()
                  ├── builds CSS + DOM (Trusted-Types-safe, pure DOM APIs)
                  ├── wireMode()      — Full video / A → B selector
                  ├── wirePanel()     — open/close the panel
                  ├── wireToggle()    — loop on/off pill
                  ├── wirePoints()    — Set / Clear / Reset buttons
                  ├── wireTimeline()  — draggable A, B and playhead thumbs
                  ├── wireTimeEdit()  — click a time to type it manually
                  ├── wireKeyboard()  — A / B shortcuts
                  ├── frame()         — RAF loop, enforces A/B boundary
                  └── wireUpdate()    — async GitHub version fetch

Navigation between YouTube videos is detected by three independent layers, all funnelled into a debounced onNav(): the native yt-navigate-finish event, a setInterval URL poll (reliable in any execution context), and a MutationObserver on <title> as a lightweight fallback. On each navigation teardown() runs — it aborts the session's AbortController (removing all global listeners) and cancels the RAF — then a fresh isolated session object is built, so there's no stale state or listener leak between videos.

Development

# Clone
git clone https://github.com/Black0S/Youtube-Loop-UserScript-.git

# Edit
# youtube-loop.js is a single self-contained file — no build step required.

# Test
# Install locally via ScriptCat or Tampermonkey, open any YouTube video.

When releasing a new version:

Bump @version in the header (e.g. 1.0.0 → 1.1.0)
Bump VERSION constant to match
Push to main — ScriptCat, Tampermonkey, and the in-panel banner will all pick it up automatically

Changelog

v3.0.0

New — Manual time entry: click a point's time to edit it in place. Strict m:ss / h:mm:ss parsing, value clamped to [0, duration]; Enter/blur confirms, Esc or an invalid value cancels, an empty value clears the point.
Fix — Listener leak: every global (document/window) listener now registers against a per-session AbortController, so teardown() removes them on SPA navigation. Previously they accumulated on every video change.
Fix — Update check: semverGt now compares correctly for 2-segment versions (e.g. 2.0 vs 2.0.0).
Perf: the mini-timeline only redraws while the panel is open; loop enforcement keeps running either way.
Cleanup: removed dead code and de-duplicated point logic into a shared applyPoint() used by Set, drag and manual edit.