main.js

import * as Comlink from "https://unpkg.com/comlink/dist/esm/comlink.mjs";

Comlink.transferHandlers.set("WORKERSQLPROXIES", {
  canHandle: obj => false,
  serialize(obj) {
    const { port1, port2 } = new MessageChannel();
    Comlink.expose(obj, port1);
    return [port2, [port2]];
  },
  deserialize: (port) => {
    port.start();
    return Comlink.wrap(port);
  }
});

const worker = new Worker("js/worker.js");
const sql = Comlink.wrap(worker);

try {
    db = await sql.new((new URL(conf['url'], window.location)).href);

    //Get all table names from master table
    tables = await db.prepare("SELECT * FROM sqlite_master WHERE type IN ('table', 'view') ORDER BY name");

    while (await tables.step()) {
        var rowObj = await tables.getAsObject();
        console.log("Got rowObj", rowObj)
    }
} finally {
    await tables.free();
}

worker.js

importScripts("https://unpkg.com/comlink/dist/umd/comlink.js");
importScripts("vendor/sqljs-wasm/sql-wasm.js");

function initTransferHandlers(sql) {
  Comlink.transferHandlers.set("WORKERSQLPROXIES", {
    canHandle: obj => {
      let isDB = obj instanceof sql.Database;
      let hasDB = obj.db && (obj.db instanceof sql.Database);
      return isDB || hasDB;
    },
    serialize(obj) {
      const { port1, port2 } = new MessageChannel();
      Comlink.expose(obj, port1);
      return [port2, [port2]];
    },
    deserialize: (port) => {
      port.start();
      return Comlink.wrap(port);
    }
  });
}

function dbFromUrl(sql, url) {
  return new Promise((res, rej) => {
    var xhr = new XMLHttpRequest();
    xhr.open('GET', decodeURIComponent(url), true);
    xhr.responseType = 'arraybuffer';

    xhr.onload = function(e) {
      if (this.status !== 200) {
        rej(e);
      }
      let arrayBuffer = this.response;
      res(new sql.Database(new Uint8Array(arrayBuffer)));
    };
    xhr.onerror = function (e) {
      rej(e);
    };
    xhr.send();
  });
}

const mod = {
  async new(url) {
    const sql = await initSqlJs({
      locateFile: file => "vendor/sqljs-wasm/" + file
    });

    initTransferHandlers(sql);
    return dbFromUrl(sql, url);
  }
};
 
Comlink.expose(mod);

Just a caution that Comlink proxies currently leak resources if you're not careful to call the [Comlink.releaseProxy] method on each one. Since you do take care to call tables.free() in main.js you probably also want to release the proxy there as well.

Thanks for the info. Good to know!

Yeah thinking about it, ComLink seems like potentially a nice abstraction, but until WeakRefs come along, it's such a leaky one that there's a pretty good argument that dealing directly with postMessage is less hassle. That seems to be the conclusion that sql.js discussion is heading towards sql-js/sql.js#377