[PROJ] proj.js: JavaScript bindings for PROJ - both Node.js native & browser WASM

Sun Jan 4 10:27:39 PST 2026

There is indeed no solution for curl in WASM, so the network must be 
reimplemented. This is what I meant when I said that there were people 
working on this and that I was going to integrate their work.

However, currently, enabling pthread support in emscripten results in a 
WASM binary that must have COOP/COEP enabled to work and this 
restriction will be passed on to any user website that uses PROJ.

Many low-end hosting providers do not support this. For example, Github 
Pages does not allow it.

There is a trick with a service worker that allows to manually override 
the flag, but this also adds a very impractical layer to every user of 
the library.

When PROJ 9.8.0 is released, with this feature, I will certainly 
integrate it in the WASM bindings, but I am still not very sure how. I 
will probably have two builds, one in which the WASM can be used as a 
normal JavaScript npm module without any restrictions, and one which has 
the network capability, but adds a restriction that will be passed down 
to every user and website that uses PROJ.

The current situation with SharedArrayBuffer is the result of the 
mitigation of a very serious security flaw (side-channel timing attacks 
- such as Meltdown and Spectre) and was introduced as an immediate 
solution in order to block these attacks. Ever since there have been 
discussions about bringing it back with some kind of security. Should 
this happen, shared memory multithreading will once again be freely 
available in WASM for everyone.

On 04/01/2026 17:39, Javier Jimenez Shaw wrote:
> Hi.
>
> You mention that there is no solution for curl in wasm.
> Since this PR https://github.com/OSGeo/PROJ/pull/4627 (in master, not 
> released yet) you can do network requests. You have to enable the 
> proper compilation flag.
>
> The js function, as explained in the PR, has to be called in a worker 
> (thread). That is an emscripten limitation.
>
> There is an example in 
> https://jjimenezshaw.github.io/wasm-proj/example.html
>
>
> I hope projinfo will be available soon as well.
>
>
> On Sun, 4 Jan 2026, 17:21 Momtchil Momtchev via PROJ, 
> <proj at lists.osgeo.org> wrote:
>
>     I am excited to announce the public availability of what should be a
>     usable beta version of proj.js - JavaScript bindings for PROJ.
>
>
>     https://github.com/mmomtchev/proj.js
>
>     https://www.npmjs.com/package/proj.js
>
>
>     proj.js is a dual-environment npm package that works both in the
>     browser
>     (compiled to WASM) and in Node.js (compiled as a native module for
>     Linux, Windows and macOS).
>
>     It exports both the new C++ API and the older C API to JavaScript
>     in two
>     separate modules.
>
>     The package is bundler-friendly and leverages Node.js 16 exports to
>     automatically load either the native version or the WASM version. It
>     comes with prebuilt binaries for WASM, Linux x86, Windows x86,
>     macOS x86
>     and macOS arm8.
>
>     Currently the only documentation are the unit tests and the
>     TypeScript
>     definitions. All methods are covered by TypeScript definitions
>     which can
>     be used as online help in IDEs that parse them. With very few
>     exceptions, all JS methods match the C and C++ API 1:1.
>
>     This is a new generation of C/C++ to JS project that leverages
>     three new
>     important technologies in the JavaScript world:
>     * SWIG JSE which renders possible the automatic generation of high
>     quality native-feel wrappers with minimal code (2000 lines of SWIG
>     code
>     for 200k lines of C/C++ code, an impressive 1:100 reduction)
>     * emnapi which renders all Node-API modules compatible with WASM
>     using
>     the same API
>     * hadron which renders possible the cross-platform compilation of
>     complex C/C++ projects for JavaScript
>
>     The whole project is entirely synchronous JavaScript on purpose -
>     most
>     of the methods are fast enough to be used on the main loop and adding
>     async support - which is a simple flip-switch in SWIG - would add
>     a very
>     restrictive requirement to the WASM module -
>     https://web.dev/articles/coop-coep - that is best avoided unless
>     there
>     is a good reason for it.
>
>     The current version is available on npm and should be more or less
>     usable. The unit testing is somewhat incomplete, but the nominal
>     codepath of all SWIG typemaps have been tested at least once on each
>     platform, including an ASAN build.
>
>     Currently the main issue is the size of the WASM bundle which
>     renders it
>     impractical for most websites. Currently, the absolute minimum is
>     about
>     1.2MB w/o libtiff, w/o proj.db, after compression and using only
>     the C
>     API. There are various options for optimising this size, but bear in
>     mind, that proj.js will always be significantly larger than proj4js.
>
>     This project, together with magickwand.js (ImageMagick for JS) -
>     which
>     is my basic tutorial, will be part of the SWIG JSE tutorials.
>     proj.js is
>     about advanced SWIG JSE techniques - the C++ API contains many modern
>     C++>=11 features and expressing the C API in a garbage-collected
>     language is not very straightforward. magickwand.js has a 1:400 code
>     reduction ratio.
>
>     You should know that I am not a very advanced PROJ user and my main
>     interest is SWIG (though I am a user), but:
>
>     * All stability problems, including memory leaks, will be thoroughly
>     investigated, in the limits of what my current access to hardware
>     allows
>     * If there are methods that are incorrectly wrapped, I will fix them
>     * I will fix bundler problems, this is something that is expected to
>     work well, I currently have examples/testing set for the major
>     bundlers
>     such as webpack, vite, rollup and the now obsolete create-react-app
>     * I do not plan to re-implement any of PROJ’ missing features in
>     WASM -
>     mainly the file and the network API - but if someone else does (I saw
>     there are people working on it), I will integrate it
>     * I do not plan to work on reducing PROJ’ own size, but I may
>     eventually
>     add emscripten split module support in order to reduce the initial
>     bundle
>
>     The current beta:
>
>     * WASM must be built with emscripten 4.0.8
>
>     https://github.com/mmomtchev/hadron/issues/79
>
>         Will be fixed in the next hadron release.
>
>     * Leaks memory if loaded/unloaded repeatedly in a Node.js
>     worker_thread
>
>     https://github.com/nodejs/node/issues/45088
>
>         As the root cause is in Node-API itself, this won't be fixed
>     in the
>     near future, I am looking at alternative solutions. Currently
>     every time
>     you create a worker_thread that uses proj.js, when it quits, part
>     of the
>     memory is not freed until the main thread exits.
>
>
>
>     -- 
>     Momtchil Momtchev <momtchil at momtchev.com>
>
>     _______________________________________________
>     PROJ mailing list
>     PROJ at lists.osgeo.org
>     https://lists.osgeo.org/mailman/listinfo/proj
>
-- 
Momtchil Momtchev<momtchil at momtchev.com>
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