[API reference](https://evolu.dev/docs/api-reference) › [@evolu/common](https://evolu.dev/docs/api-reference/common) › [Type](https://evolu.dev/docs/api-reference/common/Type) › Base64Url

```ts
const Base64Url: brand(
  "Base64Url",
  String,
  (value: string): Result<string, Base64UrlError> => {
    // Round-trip validation ensures consistency across different base64url
    // implementations (Node.js Buffer, native browser API, manual fallback).
    // Only strings that decode and encode identically are accepted.
    let roundTrip;
    try {
      roundTrip = uint8ArrayToBase64Url(
        base64UrlToUint8Array(value as Base64Url),
      );
    } catch {
      //
    }
    return roundTrip === value
      ? ok(value)
      : err<Base64UrlError>({ type: "Base64Url", value });
  },
);
```

Defined in: [packages/common/src/Type.ts:1569](https://github.com/evoluhq/evolu/blob/e7144e2bbe9069362b62dec1b64a8aa922b8f1b0/packages/common/src/Type.ts#L1569)

Base64Url without padding.

Encode with [uint8ArrayToBase64Url](https://evolu.dev/docs/api-reference/common/Type/variables/uint8ArrayToBase64Url), decode with
[base64UrlToUint8Array](https://evolu.dev/docs/api-reference/common/Type/variables/base64UrlToUint8Array).