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reverse-engineer-js

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Updated 10-16 02:28

About this skill

Reverse Engineer JS

LLM-assisted deobfuscation of minified / mangled JavaScript using humanifyjs plus surrounding tooling for extracting and reintegrating modules.

Tools at a glance
Tool Purpose When
humanifyjs (npx) Rename mangled vars/functions via LLM Mangled but already valid JS
prettier / js-beautify Reformat single-line minified code Before humanify, if needed
webcrack Unpack webpack / browserify bundles into per-module files When bundle is one big file
Source-map sleuthing // # sourceMappingURL=... may exist next to the bundle Always check first — beats LLM rename

Always check for a sourcemap (bundle.js.map or sourceMappingURL comment in the file) before spending tokens on humanify. If found, original sources are recoverable for free.

Default humanify invocation

Use Gemini unless the user says otherwise:

npx humanifyjs gemini -m "gemini-3.8-flash" -o <outputDir> <inputFile>

Requires GEMINI_API_KEY in env (the project's root .env is the usual place).

Other backends:

# OpenAI
npx humanifyjs openai --apiKey="$OPENAI_API_KEY" --model gpt-4.1-mini bundle.js

# Local (slower, free)
npx humanifyjs local bundle.js
Critical gotchas

1. Pseudo-TTY required in non-interactive shells

humanify calls process.stdout.cursorTo(...) for its progress bar. In Claude Code / CI / piped contexts, stdout is not a TTY and the call crashes with TypeError: process.stdout.cursorTo is not a function.

Wrap the call in script -q /dev/null:

script -q /dev/null npx humanifyjs gemini -m "gemini-3.8-flash" -o "$OUT" "$IN"

(macOS/BSD script syntax. On Linux: script -qc "<cmd>" /dev/null.)

2. Output filename is fixed

humanify always writes its result as <outputDir>/deobfuscated.js regardless of input filename. Don't expect module_1234.js in → module_1234.js out.

3. Long files = long runs

humanify processes top-level declarations sequentially with one LLM call each. A 5 MB bundle can take 30+ minutes. Strategies:

  • Run on single modules extracted from the bundle, not the whole thing
  • Pre-split with webcrack first if the bundle is webpacked
  • Pick the cheapest model that still reads the code well. gemini-3.8-flash is the default here; on a very large bundle a lite variant (gemini-3.5-flash-lite) cuts the per-declaration cost, at some accuracy cost on dense code

4. Keep an archive of the original

Before any rewrite, copy the pristine bundle to archive/bundle.original.js. humanify is non-deterministic and you may want to re-run with a different model.

Workflow A: Whole-bundle deobfuscation

When the bundle is small (<500 KB) or you have time:

mkdir -p archive out
cp bundle.js archive/bundle.original.js
script -q /dev/null npx humanifyjs gemini \
  -m "gemini-3.8-flash" \
  -o out bundle.js
# result: out/deobfuscated.js

Then optionally run prettier --write out/deobfuscated.js for nicer formatting.

Workflow B: Single-module extraction (large bundles)

Common in fxhash / generative-art bundles: a single bundle contains many id: (params) => { ... } modules and you only want to humanify a few interesting ones.

  1. Identify the module ID — search the bundle for distinctive strings, then trace back to the enclosing <id>: (e, t, n) => { opener.
  2. Extract the module body — match the opener, count braces (respecting strings) until the matching }.
  3. Wrap as a standalone arrow — write (e, t, n) => { <body> } to a temp file. humanify needs valid top-level code.
  4. Run humanify — script -q /dev/null npx humanifyjs gemini -m "gemini-3.8-flash" -o "$OUT" "$IN".
  5. Unwrap — strip the (e, t, n) => { ... } shell back off the result.
  6. Save and stub — write the deobf'd body to modules/module_<id>_<name>.js, replace the body in the bundle with a stub like <id>: (e, t, n) => { window.DEOBF[<id>](e, t, n); }.

If the project already has a scripts/deobfuscate.js (or similar), reuse it — the brace-counting / string-escape logic is finicky enough that re-deriving it for each new bundle is a waste. A typical script: takes a numeric module ID, extracts the body from bundle.js, runs humanify on the wrapped function, unwraps, and writes modules/module_<id>_<name>.js.

Workflow C: Webpack bundle → per-module files

When dealing with a webpack bundle, webcrack splits modules cleanly before humanify even runs:

npx webcrack bundle.js -o webcracked/
# Then humanify each module of interest:
script -q /dev/null npx humanifyjs gemini \
  -m "gemini-3.8-flash" \
  -o humanified webcracked/modules/123.js
Examples

Example 1: deobfuscate a small fxhash bundle

User says: "humanify the bundle.js in this fxhash project"

Actions:

  1. Check for sourcemap: head -c 4096 bundle.js | tail -c 200 — look for sourceMappingURL. None? Continue.
  2. Confirm GEMINI_API_KEY in .env. Source it: set -a; source .env; set +a.
  3. Archive: mkdir -p archive && cp bundle.js archive/bundle.original.js.
  4. Run: mkdir -p out && script -q /dev/null npx humanifyjs gemini -m "gemini-3.8-flash" -o out bundle.js.
  5. Beautify: npx prettier --write out/deobfuscated.js.

Result: out/deobfuscated.js with renamed vars, ready to read.

Example 2: deobfuscate one module out of a giant bundle

User says: "Deobfuscate module 3658 from og/bundle.js, name it treeGenerator"

Actions:

  1. If the project already has a deobfuscation helper (e.g. scripts/deobfuscate.js), run it: node scripts/deobfuscate.js 3658 treeGenerator.
  2. Otherwise: extract the module body by brace-counting, wrap as (e, t, n) => { ... }, run humanify with script -q /dev/null, unwrap, save to modules/module_3658_treeGenerator.js, stub the bundle.

Result: New file in modules/ with a readable version of the function; bundle.js shrunk by stubbing.

Example 3: humanify crashed with cursorTo is not a function

User says: "humanify just dies immediately"

Diagnosis: Non-TTY stdout (this is the default in Claude Code's bash tool).

Fix: Re-run wrapped in script -q /dev/null <cmd> (macOS) or script -qc "<cmd>" /dev/null (Linux).

Troubleshooting

cursorTo is not a function

Cause: stdout is not a TTY. Fix: wrap in script -q /dev/null.

GEMINI_API_KEY not set

Cause: humanify reads from process env, not from the project .env automatically. Fix: set -a; source .env; set +a in the same shell, or pass inline: GEMINI_API_KEY=... npx humanifyjs gemini ....

Output is identical to input (no renames)

Cause: humanify only renames identifiers it considers mangled (short, single-letter). Already-readable code is left alone. Fix: confirm the input is actually obfuscated; if names like useState survive that's expected, not a bug.

Rate limits / quota errors from Gemini

Cause: large file, many top-level functions → many sequential LLM calls. Fix: switch to local backend for free local rename, or extract a single module (Workflow B) instead of the whole bundle.

humanify takes hours

Cause: big bundle, sequential per-function calls. Fix: pre-split with webcrack, or only humanify the modules you actually need.

Output directory contains stale deobfuscated.js

Cause: humanify writes to a fixed filename. A failed second run will pick up the previous good output. Fix: rm -f $OUT/deobfuscated.js before each run.