svelteesp32

Convert Svelte (or React/Angular/Vue) JS application to serve it from ESP32 webserver

Forget SPIFFS and LittleFS now

I often make small to medium-sized microcontroller solutions that run on ESP32. If a web interface is needed, I create a Svelte application. The Svelte application is practically served by the ESP32.

In order to be able to easily update OTA, it is important - from the users' point of view - that the update file consists of one file. I can't use the SPIFFS/LittleFS solution for this. It is necessary that the WebUI files are included inline in the Arduino or PlatformIO c++ code.

This npm package provides a solution for inserting any JS client application into the ESP32 web server (PsychicHttp is my favorite, faster than ESPAsyncWebServer). For this, JS, html, css, font, assets, etc. files must be converted to binary byte array. Npm mode is easy to use and easy to integrate into your CI/CD pipeline.

Starting with version v1.1.0, the ETag header is also supported.

Usage

Install package as devDependency (it is practical if the package is part of the project so that you always receive updates)

npm install -D svelteesp32

After a successful Svelte build (rollup/webpack/vite) create an includeable c++ header file

npx svelteesp32 -s ../svelteapp/dist -o ../esp32project/svelteesp32.h --etag

During the translation process, the processed file details are visible, and at the end, the result shows the ESP32's memory allocation (gzip size)

[assets/index-KwubEIf-.js]
✓ gzip used (38850 -> 12547)

[assets/index-Soe6cpLA.css]
✓ gzip used (32494 -> 5368)

[favicon.png]
x gzip unused (33249 -> 33282)

[index.html]
✓ gzip used (472 -> 308)

[roboto_regular.json]
✓ gzip used (363757 -> 93567)

5 files, 458kB original size, 142kB gzip size
../../../Arduino/EspSvelte/svelteesp32.h 842kB size

Include svelteesp32.h into your Arduino or PlatformIO c++ project (copy it next to the main c++ file)

...

#include <PsychicHttp.h>
#include "svelteesp32.h"

PsychicHttpServer server;

...

void setup()
{
    ...

    server.listen(80);

    initSvelteStaticFiles(&server);
}

The content of generated file (do not edit, just use)

const uint8_t data0[12547] = {0x1f, 0x8b, 0x8, 0x0, ...
const uint8_t data1[5368] = {0x1f, 0x8b, 0x8, 0x0, 0x0, ...
const char * etag0 = "387b88e345cc56ef9091...";

void initSvelteStaticFiles(PsychicHttpServer * server) {
  server->on("/assets/index-KwubEIf-.js", HTTP_GET, [](PsychicRequest * request)
  {
    if (request->hasHeader("If-None-Match") && ...) {
      PsychicResponse response304(request);
      response304.setCode(304);
      return response304.send();
    }

    PsychicResponse response(request);
    response.setContentType("application/javascript");
    response.addHeader("Content-Encoding", "gzip");
    response.setContent(data0, sizeof(data0));
    return response.send();
  });

  server->on("/assets/index-Soe6cpLA.css", HTTP_GET, [](PsychicRequest * request)
  {
    if (request->hasHeader("If-None-Match") && ...) {
      ...
    }

    PsychicResponse response(request);
    response.setContentType("text/css");
    response.addHeader("Content-Encoding", "gzip");
    response.setContent(data1, sizeof(data1));
    return response.send();
  });

    ...
}

Gzip

All modern browsers have been able to handle gzip-compressed content for years. For this reason, there is no question that the easily compressed JS and CSS files are stored compressed in the ESP32 and sent to the browser.

During the translation process, data in gzip format is generated and will be used if the size is greater than 100 bytes and we experience a reduction of at least 15%. In such a case, the compressed data is unconditionally sent to the browser with the appropriate Content-Encoding header information.

Automatic compression can be turned off with the --no-gzip option.

ETag

The ETag HTTP header can be used to significantly reduce network traffic. If the server sends ETag information, the client can check the integrity of the file by sending back this ETag (in If-None-Match header) without sending the data back again. All browsers use this function, the 304 HTTP response code is clearly visible in the network traffic.

Since ESP32 data traffic is moderately expensive, it is an individual decision whether to use the ETag or not. We recommend using ETag, which adds a bit more code (about 1-3%) but results in a much cleaner operation.

The use of ETag is not enabled by default, this can be achieved with the --etag switch.

Main entry point - index.html

Typically, the entry point for web applications is the index.htm or index.html file. This does not need to be listed in the browser's address bar because web servers know that this file should be served by default. Svelteesp32 also does this: if there is an index.htm or index.html file, it sets it as the main file to be served. So using http://esp32_xxx.local or just entering the http://x.y.w.z/ IP address will serve this main file.

Command line options

Option	Required	Description	default
-s	x	Source dist folder contains compiled web files
-o	x	Generated output file with path	svelteesp32.h
--etag		Use ETag header for cache	false
--no-gzip		Do not compress content with gzip	false -> gzip used
--espmethod	x	Name of generated method	initSvelteStaticFiles
-h		Show help

Q&A

• How big a frontend application can be placed? If you compress the content with gzip, even a 3-4Mb assets directory can be placed. This is a serious enough amount to serve a complete application.

• How fast is cpp file compilation? The cpp file can be large, but it can be compiled in a few seconds on any machine. If you don't modify your svelte/react app, it will use the already compiled cpp file (not recompile). This does not increase the speed of ESP32 development.

• Does the solution use PROGMEM? No and yes. ESP32 no longer has PROGMEM. (Exists, but does not affect the translation). Instead, if we use a const array in the global namespace, its content will be placed in the code area, i.e. it will not be used from the heap or the stack, so the content of the files to be served will be placed next to the code.

• Is this safe to use in production? I suggest you give it a try! If you find it useful and safe in several different situations, feel free to use it, just like any other free library.

• Will you develop it further? Since I use it myself, I will do my best to make the solution better and better.