feat: add generateDocumentId utility for random Firestore document IDs

.changeset/generate-firestore-id.md

@@ -0,0 +1,5 @@
+---
+'fireworkers': minor
+---
+
+Add `generateDocumentId()`, a utility that returns a random 20-character `[A-Za-z0-9]` ID in the same format Firestore uses for auto-generated document IDs. Useful when you need to know a document's ID before writing it (e.g. to reference it from sibling writes in a `batch`). Ported from `@firebase/firestore`'s `AutoId.newId()` — uses `crypto.getRandomValues` with rejection sampling to avoid modulo bias.

README.md

@@ -339,6 +339,23 @@ const response = await b.commit();
 
 ---
 
+### generateDocumentId()
+
+Generates a random ID matching Firestore's auto-generated document ID format: 20 characters from `[A-Za-z0-9]`, produced with rejection sampling via `crypto.getRandomValues` to avoid modulo bias. Ported from [`@firebase/firestore`'s `AutoId.newId()`](https://github.com/firebase/firebase-js-sdk/blob/main/packages/firestore/src/util/misc.ts).
+
+Useful when you need to know a document's ID before writing it — for example, to reference it from other documents in the same [`batch`](#batchdb).
+
+```typescript
+const id = Firestore.generateDocumentId();
+
+const b = Firestore.batch(db);
+b.set(['todos', id], { title: 'Win the lottery', completed: false });
+b.set(['todo-index', id], { createdAt: Date.now() });
+await b.commit();
+```
+
+---
+
 ## Error handling
 
 All operations reject with a `FirestoreError` when Firestore returns an error response or the network request fails. `FirestoreError` extends the built-in `Error`, so existing `try/catch` and `.message` checks keep working — but you can now branch on a stable string `code` instead of parsing the message.

src/id.test.ts

@@ -0,0 +1,16 @@
+import { describe, expect, it } from 'vitest';
+
+import { generateDocumentId } from './id';
+
+describe('generateDocumentId', () => {
+  it('returns a 20-character string', () => {
+    const id = generateDocumentId();
+    expect(id).toHaveLength(20);
+  });
+
+  it('only uses characters from [A-Za-z0-9]', () => {
+    for (let i = 0; i < 100; i++) {
+      expect(generateDocumentId()).toMatch(/^[A-Za-z0-9]{20}$/);
+    }
+  });
+});

src/id.ts

@@ -0,0 +1,31 @@
+const AUTO_ID_CHARS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
+const AUTO_ID_LENGTH = 20;
+// Largest multiple of AUTO_ID_CHARS.length that fits in a byte.
+// Bytes at or above this value are discarded so the modulo below is unbiased.
+const MAX_MULTIPLE = Math.floor(256 / AUTO_ID_CHARS.length) * AUTO_ID_CHARS.length;
+// Over-allocate to amortize rejected bytes: acceptance rate is ~97% (248/256),
+// so 2× AUTO_ID_LENGTH almost always fills the ID in a single iteration.
+const RANDOM_BYTES_PER_ITERATION = AUTO_ID_LENGTH * 2;
+
+/**
+ * Generates a random ID matching Firestore's auto-generated document ID format.
+ * 20 characters from [A-Za-z0-9], with rejection sampling to avoid modulo bias.
+ * Ported from `@firebase/firestore`'s `AutoId.newId()`.
+ */
+export const generateDocumentId = (): string => {
+  let id = '';
+
+  while (id.length < AUTO_ID_LENGTH) {
+    const bytes = new Uint8Array(RANDOM_BYTES_PER_ITERATION);
+    crypto.getRandomValues(bytes);
+
+    for (let i = 0; i < bytes.length && id.length < AUTO_ID_LENGTH; i++) {
+      const byte = bytes[i]!;
+      if (byte < MAX_MULTIPLE) {
+        id += AUTO_ID_CHARS.charAt(byte % AUTO_ID_CHARS.length);
+      }
+    }
+  }
+
+  return id;
+};

src/index.ts

@@ -2,6 +2,7 @@ export * from './batch';
 export * from './create';
 export { FirestoreError, type FirestoreErrorCode } from './error';
 export * from './get';
+export * from './id';
 export * from './init';
 export * from './query';
 export * from './remove';