add isClosed getter (#238)

* add `isClosed` getter
* add tests and documentation to `isClosed` method

src/db.test.ts

@@ -215,6 +215,13 @@ Deno.test("close database", function () {
   db.close(); // check close is idempotent and won't throw
 });
 
+Deno.test("check database is closed", function () {
+  const db = new DB();
+  assertEquals(db.isClosed, false);
+  db.close();
+  assertEquals(db.isClosed, true);
+});
+
 Deno.test("open queries block close", function () {
   const db = new DB();
   db.query("CREATE TABLE test (name TEXT PRIMARY KEY)");

src/db.ts

@@ -737,4 +737,12 @@ export class DB {
   get autoCommit(): boolean {
     return this.#wasm.autocommit() !== 0;
   }
+
+  /**
+   * Returns `true` when the database handle is closed
+   * and can no longer be used.
+   */
+  get isClosed(): boolean {
+    return !this.#open;
+  }
 }