JS9 Deployment Modes: Client-side vs. Helper

Deployment Modes: Client-side vs. Server Helper

JS9 can run in two different modes, and you choose which one by a single preference setting. Both modes display and analyze FITS images; they differ only in where the heavier, optional work happens.

The short version

Client-side (static) — everything runs in the browser. Deploy the files to any static host (GitHub Pages, S3, a plain web server, even a file:// path with a local server). No back end. This is the default.
Server helper — a small Node.js program (js9Helper.js) runs alongside the static files and adds server-side capabilities.

How the analysis actually works

JS9's FITS engine is astroem — CFITSIO, WCS and related libraries compiled to WebAssembly. It runs inside the browser. That means the core of JS9, including a large amount of analysis, needs no server at all:

load and display FITS (images, tables, cubes, multi-extension files)
colormaps, scaling, zoom/pan, RGB, blending
WCS: pixel ↔ world coordinates, changing WCS system/units
regions, including the boolean selection parser and grouping
region statistics, counts in regions, radial profiles, histograms, encircled energy, pixel tables, projections (the imexam plugins)
image arithmetic, gaussian blur, filtering, binning, reprojection, rotation

What the helper adds (and how to live without it)

Capability	Helper	Static alternative
Server-side analysis tasks (run external programs, e.g. funtools)	yes	write client-side "local" tasks in JavaScript
Proxy load of remote FITS from a non-CORS server	yes	host the FITS yourself, or use a CORS-enabled archive (direct URL loads work)
Saving files / per-user work directories on the server	yes	browser downloads

Choosing a mode

The mode is set by globalOpts.helperType in js9prefs.js (loaded by your web page):

  // client-side / static (default): no back end
  "helperType": "none",
  "loadProxy":  false,

  // server helper: adds server-side analysis + proxy
  "helperType": "nodejs",
  "loadProxy":  true,

See Preferences for the full list of options and Server-side Analysis and The JS9 Helper for the helper details.

Running each mode

Static: build the site and serve the output directory — npm run serve for local preview, or publish the built _site/ directory to any static host.
Helper: set helperType: "nodejs", then start the Node helper (node server/js9Helper.js) so the page can connect to it on its configured port.

If you deploy the static files but leave helperType: "nodejs", the page will try (and fail) to reach a helper that isn't there — harmless, but it logs a connection error. Set helperType: "none" for a pure static site.