das2js

Das servers typically provide data relevant to space plasma and magnetospheric physics research. To retrieve data, an HTTP GET request is posted to a das server by a client program and a self-describing stream of data values covering the requested time range, at the requested time resolution, is provided in the response body. This project aims to read those streams in web browsers by compiling das2C to WebAssembly, so parsing and value decoding happen in C and JavaScript receives typed-array views of the decoded values without a conversion pass.

Early days, but it runs. make module produces a three-file ES module in build.wasm32/ (das2js.mjs, das2c.mjs, das2c.wasm) that fetches a das stream, parses it as it arrives, and hands back TypedArray views of the decoded values, int64 and all. It has read real data from a public dasFlex server in Firefox. No package, no minified build, no catalog browser, no plotting; a progress meter, a source picker, a catalog tree view and a live spectrogram are the planned demos. Layout:

• src/ - the hand-written ES module, one file
• glue/ - a small flat C interface over das2C, the wasm export surface
• test/ - the glue test (native and wasm under node) and the browser tests (headless Firefox via a node runner)
• deps/ - das2C and expat, pinned as git submodules and compiled in
• buildfiles/ - the hand-written expat configuration for wasm32
• notes/ - build recipe, measurements

It is fast. A full day of Juno Waves burst waveforms, 505 MB of samples, parses in Firefox in under 300 ms once the bytes are in memory, about 1.7 GB/s, and the first record is usable half a second after the request goes out while the server spends the next 36 s producing the rest. No JSON on the wire, no number conversion in JavaScript: the values arrive in their native binary encoding and the browser reads them where they landed. Screenshot of the live run below. Method and more streams in notes/performance.md.

Like the other das2C bindings (das2py, das2dlm) this project compiles a pinned das2C commit in rather than linking a system copy. Until das2C v3.0.0 is tagged that commit is on das2C master. notes/build.md covers the toolchain, the build and the tests; git submodule update --init fetches the pinned sources.

This repository was started in 2022 and promptly went dormant. It is active again as of October 2026, though if you need a working das client today, das2py is the mature path and this is not yet an alternative.