HackerReposA FIELD GUIDE TO OPEN SOURCE
ProSponsor the shelf♧ 0
← Back to top all-time
justjake
quickjs-emscripten
TypeScript2020–2026
justjake / quickjs-emscripten

quickjs-emscripten

Safely execute untrusted Javascript in your Javascript, and execute synchronous code that uses async functions

Web DevelopmentSystems & Infrastructure
Explore on GitHubRead Hacker News discussions
10Distinct people
28Mentions
0Explicit recommenders
1.7KGitHub stars
REPOSITORY AT A GLANCE

What to know before exploring

What it is

Safely execute untrusted Javascript in your Javascript, and execute synchronous code that uses async functions

GitHub

Where it is used

No documented product use has been verified yet.

What people are saying

No recommendation phrase was found in the indexed comments.

No classified warning is attached to this page. Read the discussions before treating silence as approval.

Also in these discussions

No other repository is repeatedly linked in the same discussions.

This week on Hacker News

Nov 7–Nov 14 UTC · Direct repository links and separately labeled related project stories. Indexed through 11/14/2026, 12:54:37 PM UTC.

No direct repository links were found in the indexed HN discussions over the past seven days. Older discussions are available below.

Mentions over time

HN mentions by year28 dated mentions

Observed mentions: 2020-01-14 17:25:52 UTC to 2026-02-14 17:59:12 UTC.

0369122020-01-01 00:00:00 UTC to 2021-01-01 00:00:00 UTC (exclusive): 3 mentions; 1 submissions, 2 comments20202021-01-01 00:00:00 UTC to 2022-01-01 00:00:00 UTC (exclusive): 3 mentions; 0 submissions, 3 comments2022-01-01 00:00:00 UTC to 2023-01-01 00:00:00 UTC (exclusive): 11 mentions; 1 submissions, 10 comments2023-01-01 00:00:00 UTC to 2024-01-01 00:00:00 UTC (exclusive): 4 mentions; 0 submissions, 4 comments20232024-01-01 00:00:00 UTC to 2025-01-01 00:00:00 UTC (exclusive): 4 mentions; 0 submissions, 4 comments2025-01-01 00:00:00 UTC to 2026-01-01 00:00:00 UTC (exclusive): 2 mentions; 0 submissions, 2 comments2026-01-01 00:00:00 UTC to 2027-01-01 00:00:00 UTC (exclusive): 1 mentions; 0 submissions, 1 comments. Incomplete calendar period.2026*

* An incomplete calendar period is marked with an outlined bar. Its count is not directly comparable with a full period.

Counts come from the repository’s dated source evidence. Index cutoff: 2026-11-02 08:16:59 UTC.

View mention counts and date ranges
UTC ranges include their start and exclude their end. Select a period to filter the source evidence.
PeriodStart (UTC)End (exclusive, UTC)MentionsSubmissionsComments
20202020-01-01 00:00:00 UTC2021-01-01 00:00:00 UTC312
20212021-01-01 00:00:00 UTC2022-01-01 00:00:00 UTC303
20222022-01-01 00:00:00 UTC2023-01-01 00:00:00 UTC11110
20232023-01-01 00:00:00 UTC2024-01-01 00:00:00 UTC404
20242024-01-01 00:00:00 UTC2025-01-01 00:00:00 UTC404
20252025-01-01 00:00:00 UTC2026-01-01 00:00:00 UTC202
2026 (partial)2026-01-01 00:00:00 UTC2027-01-01 00:00:00 UTC101

What Hacker News says

28 sources
jitl·May 17, 2025
Yeah, great for that use-case - memory management; it's great to get the DisposeStack that allows "moving" out of the current scope too, that's handy. I adopted it for quickjs-emscripten (my quickjs in wasm thingy for untrusted code in the browser) but found that differing implementations between the TypeScript compiler and Babel lead to it not being reliably usable for my consumers. I ended up writing this code to try to work around the polyfill issues; my compiler will use Symbol.for('Symbol.dispose'), but other compilers may choose a different symbol... https://github.com/justjake/quickjs-emscripten/blob/aa48b619...
In JavaScript's New Superpower: Explicit Resource Management
AgentME·Dec 18, 2024
If you were not worried about preventing external requests and were okay with it being able to make requests as an arbitrary webpage unrelated to your site could if opened, then loading the code in an iframe with the sandbox="allow-scripts" attribute is enough. If you needed tighter sandboxing than that, then I think WebAssembly is your best bet because you can control exactly what APIs are exposed to a WebAssembly module. You could then let users submit WebAssembly modules or let them submit JS that will be run in a JS interpreter running in WebAssembly. I think https://github.com/justjake/quickjs-emscripten or https://github.com/fermyon/StarlingMonkey (used by https://github.com/bytecodealliance/ComponentizeJS) look good for that.
In The XOR Texture (2004)
jitl·Jul 7, 2024
I’m the author of the underlying quickjs-emscripten library. It supports the browser (specifically tested with ESM.sh), as well as Cloudflare Workers, NodeJS, Deno: https://github.com/justjake/quickjs-emscripten?tab=readme-ov... It has APIs for exposing host functions, calling guest functions, custom module loaders, etc: https://github.com/justjake/quickjs-emscripten?tab=readme-ov... API docs for newFunction: https://github.com/justjake/quickjs-emscripten/blob/main/doc...
In Show HN: Execute JavaScript in a WebAssembly QuickJS sandbox
jitl·Jul 7, 2024
Hi, I’m the author of the underlying quickjs-emscripten runtime library. I like your ergonomic kind of “standard library” for quickjs-emscripten :) Did you try running in the browser or with a bundler? I think accepting the variant name as a string you pass to import(variantName) dynamically may not play well with Webpack et al. EDIT: SECURITY WARNING: this library exposes the ability for the guest (untrusted) code to `fetch` with the same cookies as the host `fetch` function. You must not run untrusted code if enabling `fetch`. Library should come with a big blinking warning about what is safe and unsafe to enable when running untrusted code. It’s not a “sandbox” if the sandboxed code can call arbitrary HTTP APIs authenticated as the host context! The reason quickjs-emscripten is low-level and avoids magic is so I can confidently claim
Read the rest
that the APIs it does provide are secure. I generally reject feature requests for magical serialization or easy network/filesystem access because that kind of code is a rich area for security mistakes. When you run untrusted code, you should carefully audit the sandbox itself, but also audit all the code you write to expose APIs to the sandbox. In this case a comment from an other HN user asking about Fetch cookies tipped me off to the potential security issue. More reading: Figma blog posts on plugin sandbox security: - https://www.figma.com/blog/how-we-built-the-figma-plugin-sys... - https://www.figma.com/blog/an-update-on-plugin-security/ Quickjs-emscripten README: https://github.com/justjake/quickjs-emscripten
In Show HN: Execute JavaScript in a WebAssembly QuickJS sandbox
jitl·Dec 9, 2023
Based on your comment below I think you figured out the difference - but if you're looking to execute untrusted Javascript from Javascript in Node or the browser, you can pick between ShadowRealm (where available, or using a polyfill) or my library quickjs-emscripten. Pros of quickjs-emscripten over ShadowRealm: - You can use quickjs today in any browser with WASM. ShadowRealm isn't available yet, and polyfills have had security issues in the past. See https://www.figma.com/blog/an-update-on-plugin-security/ - In ShadowRealm eval, untrusted code can consume arbitrary CPU cycles. With QuickJS, you can control the CPU time used during an `eval` using an [interrupt handler] that's called periodically during the eval. - In ShadowRealm eval, untrusted code can allocate arbitrary amounts of memory. With QuickJS, you can control both the [stack
Read the rest
size] and the [heap size] available inside the runtime. - quickjs-emscripten can do interesting things with custom module loaders and facades that allow synchronous code inside the runtime to call async code on the host. Pros of ShadowRealm over QuickJS: - ShadowRealm will (presumably?) execute code using your native runtime, probably v8, JavaScriptCore, or SpiderMonkey. Quickjs is orders of magnitude slower than JIT'd javascript performance of v8 etc. It's also slower than v8/JSC's interpreters, although not by a huge amount. See [benchmarks] from 2019. - You can easily call and pass values to ShadowRealm imported functions. Talking to quickjs-emscripten guest code requires a lot of fiddly and manual object building. - Overall the quickjs(-emscripten) API is verbose, and requires manual memory management of references to values inside the quickjs runtime. [interrupt handler]: https://github.com/justjake/quickjs-emscripten/blob/main/doc... [stack size]: https://github.com/justjake/quickjs-emscripten/blob/main/doc... [heap size]: https://github.com/justjake/quickjs-emscripten/blob/main/doc... [benchmarks]: https://bellard.org/quickjs/bench.html
In New QuickJS Release
jitl·Dec 9, 2023
I just upgraded quickjs-emscripten - my JS-in-JS wrapper library - to this new version of the upstream quickjs. If you want to safely evaluate untrusted Javascript in the browser or Node with control over CPU time and memory use, a custom module system, or call async code synchronously, check it out: https://github.com/justjake/quickjs-emscripten It's used by a few projects like Hoppscotch, a Postman-like visual API client (https://hoppscotch.io/). They use it for request scripting: https://docs.hoppscotch.io/documentation/getting-started/res...
In New QuickJS Release
simonw·Oct 4, 2023
The thing I want to achieve with WebAssembly is still proving a lot harder than I had anticipated. I want to be able to take strings of untrusted code provided by users and execute them in a safe sandbox. I have all sorts of things I want this for - think custom templates for a web application, custom workflow automation scripts (Zapier-style), running transformations against JSON data. When you're dealing with untrusted code you need a really robust sandbox. WebAssembly really should be that sandbox. I'd like to support Python, JavaScript and maybe other languages too. I want to take a user-provided string of code in one of those languages and execute that in a sandbox with a strict limit on both memory usage and time taken (so I can't be crashed by a "while True" loop). If memory or time limit are exceeded, I want to get an exception wh
Read the rest
ich I can catch and return an error message to the user. I've been exploring options for this for quite a while now. The furthest I've got was running Python in wasmtime: https://til.simonwillison.net/webassembly/python-in-a-wasm-s... and running Pyodide inside of Deno: https://til.simonwillison.net/deno/pyodide-sandbox Surprisingly I've not found a good pattern for running a JavaScript interpreter in a WASM sandbox yet. https://github.com/justjake/quickjs-emscripten looks promising but I've not found the right recipe to call it from server-side Python or Deno yet. Can Extism help with this? I'm confident I'm not the only person who's looking for a solution here!
In Extism makes WebAssembly easy
jitl·Nov 19, 2022
If you need to call into user-generated Javascript synchronously or have greater control over the sandbox environment, you can use WebAssembly to run a Javascript interpreter: https://github.com/justjake/quickjs-emscripten#quickjs-emscr... QuickJS in WebAssembly is much slower than your browser's native Javascript runtime, but possibly faster than async calls using postMessage. As an added bonus, it can make async functions in the host appear to be synchronous inside the sandbox using asyncify: https://github.com/justjake/quickjs-emscripten#async-on-host...
In Show HN: Run unsafe user generated JavaScript in the browser
jitl·Nov 7, 2022
You can use One Weird Trick with generator functions to make your code "generic" over synchronicity. I use this technique to avoid needing to implement both sync and async versions of some functions in my quickjs-emscripten library. The great part about this technique as a library author is that unlike choosing to use a Promise return type, this technique is invisible in my public API. I can write a function like `export function coolAlgorithm(getData: (request: I) => O | Promise<O>): R | Promise<R>`, and we get automatic performance improvement if the caller's `getData` function happens to return synchronously, without mystery generator stuff showing up in the function signature. Helper to make a function that can be either sync or async: https://github.com/justjake/quickjs-emscripten/blob/ff211447... Uses: https://cs.github.com/justjake
Read the rest
/quickjs-emscripten?q=yield*+l...
In Why would anyone need JavaScript generator functions?

Showing the 15 most recent of 28 sources. Enable JavaScript to search and browse every indexed mention.

MAKE YOURSELF AT HOME

A few shortcuts

/
Search the collection
?
Open this guide
Esc
Close dialogs and navigation

Your saved repositories and interests stay in this browser.