A Chrome extension that turns YouTube into English reading practice: click any word in the captions and get an explanation of what it means in that sentence, then review what you kept with built-in spaced repetition.
Built for an advanced learner who reads technical English fine but keeps tripping over idioms, phrasal verbs and slang — so the explanation leads with context, not with a dictionary entry.
- Click a word in the captions → an AI explanation of that word in this sentence, plus 2–4 general senses, each with an example sentence and a Chinese gloss.
- Phrases are grouped.
grew intois one clickable box, not two words — idioms and phrasal verbs are exactly what an advanced learner misses, and boxing each word separately hides them. - Parts of speech are colour-coded. Verbs blue, nouns amber, adjectives green, prepositions pink, function words dimmed. The structure of a sentence becomes visible at a glance.
- Sentence navigation. A previous sentence, S replay, D next — real sentence boundaries, not caption chunks.
- Chinese subtitles with Z, at no API cost (YouTube's own translation).
- Listening mode with X: the English captions blur to unreadable so your ear does the work; hover a word to peek at just that word. Pair with Z for translation-only listening. Reading captions is the path of least resistance, and the brain will take it.
- Hover to freeze. Moving the mouse onto a word pauses the video so you can actually click it; moving away resumes.
- Spaced repetition built in. Mark a word 學習中, and it enters an SM-2 review queue ordered by how much of the memory has probably decayed.
No build step, no dependencies. Clone it and load it:
chrome://extensions→ enable Developer mode- Load unpacked → select this folder
- Open the extension's options page and paste an Anthropic API key
- Open a YouTube video with captions on and reload the page
It calls Claude Haiku 4.5 three ways, all batched and cached per video:
| Call | When | Roughly |
|---|---|---|
| Word explanation | Each word you click | Fractions of a cent |
| Phrase scan | Once per video | One call, cached |
| Part-of-speech tagging | Once per video | The expensive one — a few hundred lines of output |
The options page shows real token usage and estimated cost, broken down by call type. For one person watching a few videos a day this lands in cents per month, but measure rather than trust that sentence — that's what the meter is for.
The interesting problem is getting the transcript at all. YouTube gates /api/timedtext behind a
Proof-of-Origin Token, so fetching a caption track directly returns 200 with an empty body.
immerse doesn't forge a token. A MAIN-world content script monkey-patches fetch and
XMLHttpRequest.open and copies the URL of the request the player itself makes — already
signed — onto a DOM attribute, which is the only channel the isolated world can read. Re-fetching
that URL with fmt=json3 yields the whole timed transcript; the same URL with tlang=zh-Hant
yields YouTube's own translation, which is why the Chinese subtitle line costs nothing.
Having the full transcript up front is what makes everything else simple: sentence boundaries are computed once, so clicking a word gives you the complete sentence immediately rather than waiting for the speaker to finish it.
hook.js MAIN world, document_start — captures the signed timedtext URL
content.js isolated world — captions, clicking, colouring, the vocabulary deck
bg.js service worker — every network call (host_permissions exempt it from CORS)
review.html the spaced-repetition screen
options.html API key, usage meter, CSV export
node test.js # pure logic: sentence stitching, phrase matching, POS, SM-2, CSV, cost maths
node icon.js # regenerate the icons (a hand-rolled PNG encoder, zlib is the only dependency)There is deliberately no npm, no bundler and no framework. Every dependency was declined on
purpose, and the comments say which and why — ponytail: marks a shortcut with a known ceiling
and the upgrade path if it ever matters.
These are known and deliberate, not oversights:
- YouTube can break this at any time. It depends on the player's own caption request and on DOM class names. There is no SLA; when it breaks, you fix it.
- Irregular verbs may not get coloured (
grew→grow). The stemmer is naive on purpose; a real lemmatiser means npm and a bundler. - A phrase split across two caption lines falls back to separate words.
- The first sentence of a video is uncoloured for a few seconds while the tagging call returns. Cached per video, so a rewatch is instant.
- The API key is stored in plaintext in
chrome.storage.local. Anything that can read your Chrome profile can read it. - Reviews are desktop-only and don't sync. If you want mobile and sync, export to CSV and use Anki instead — that was the original plan and it's still a reasonable one.
The design owes a lot to zeroStudy, which solved this problem first and solved it well. immerse started as a teardown of it and ended up as a personal tool with a different bet: the explanation is context-first, and everything runs against your own API key.
MIT — see LICENSE.
一個把 YouTube 變成英文閱讀練習場的 Chrome 擴充：點字幕上的任何一個字，看它在這句話裡的意思， 再用內建的間隔重複複習記下來的詞。
目標使用者是讀技術英文沒問題、但常卡在慣用語、片語動詞和俚語的中高階學習者——所以解釋是 語境優先，不是先丟一頁字典義項。
- 點字看它在這句的意思，另附 2–4 個常見義項，各配英文例句和中文語意。
- 片語成組：
grew into是一個可點的框，不是兩個字。中高階卡的正是這些跨詞的東西。 - 詞性上色：動詞藍、名詞琥珀、形容詞綠、介系詞粉紅、功能詞灰掉。
- 句子導航：A 上一句、S 重播、D 下一句——切在真正的句子邊界。
- 中文對照字幕（Z），零 API 成本，用的是 YouTube 自己的翻譯。
- 進階聽力模式（X）：英文字幕模糊到不可讀，逼耳朵工作；滑到單一個字上偷看那個字。 搭配 Z 就是「只看翻譯」的聽力訓練。依賴文字是聽力的殺手。
- 滑到字上就暫停，移開繼續，不用先按空白鍵。
- 間隔重複複習：按下「學習中」才進詞彙庫，佇列依「記憶衰退程度」排序。
不需要建置、沒有相依套件。
chrome://extensions→ 開啟開發人員模式- 載入未封裝項目 → 選這個資料夾
- 到擴充功能選項貼上 Anthropic API key
- 開一支有開字幕的 YouTube 影片，重新整理
沿用 zeroStudy 的建議：每小時沉浸最多標 10 個字。標太多會累積成清不完的待複習。超過時彈窗會提醒， 但不會阻止你——那是你的判斷。時數用的是「影片實際播放且分頁在前景」的秒數。
見上方 Known limitations。最重要的一條：YouTube 隨時可能改版打壞它， 壞了要自己修，沒有 SLA。