Add an SqliteWorker

src/workers.ts

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+import SqliteWorker from './workers/sqlite.ts';
+
+const sqliteWorker = new SqliteWorker('./data/db.sqlite3');
+
+export { sqliteWorker };

src/workers/sqlite.ts

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+class SqliteWorker {
+  #path: string;
+  #worker: Worker;
+
+  constructor(path: string) {
+    this.#path = path;
+    this.#worker = new Worker(new URL('./sqlite.worker.ts', import.meta.url).href, { type: 'module' });
+  }
+
+  open(): Promise<void> {
+    return this.#call(['open', [this.#path]]);
+  }
+
+  query(sql: string, params?: any): Promise<unknown[]> {
+    return this.#call(['query', [sql, params]]);
+  }
+
+  #call<T>(msg: [string, unknown[]]): Promise<T> {
+    const id = crypto.randomUUID();
+
+    this.#worker.postMessage([id, msg]);
+
+    // TODO: use a hashmap instead of an event listener for better performance.
+    return new Promise((resolve) => {
+      const handleEvent = (event: MessageEvent<[string, T]>) => {
+        const [_id, result] = event.data;
+        if (_id === id) {
+          this.#worker.removeEventListener('message', handleEvent);
+          resolve(result);
+        }
+      };
+      this.#worker.addEventListener('message', handleEvent);
+    });
+  }
+}
+
+export default SqliteWorker;

src/workers/sqlite.worker.ts

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+/// <reference lib="webworker" />
+
+import { DenoSqlite3 } from '@/deps.ts';
+
+let db: DenoSqlite3;
+
+type Msg =
+  | ['open', [string]]
+  | ['query', [string, unknown[]]];
+
+function call([cmd, args]: Msg) {
+  switch(cmd) {
+    case 'open':
+      return handleOpen(args[0]);
+    case 'query':
+      return handleQuery(args[0], args[1]);
+  }
+}
+
+function handleOpen(path: string): void {
+  db = new DenoSqlite3(path);
+}
+
+function handleQuery(sql: string, params: any[] = []) {
+  return db.prepare(sql).all(...params);
+}
+
+self.addEventListener('message', (event: MessageEvent<[string, Msg]>) => {
+  const [id, msg] = event.data;
+  const result = call(msg);
+  self.postMessage([id, result]);
+});