feat: add isClient guard

__tests__/index.test.ts

@@ -1,5 +1,5 @@
 import SqlString from 'sqlstring'
-import { connect, format, hex, DatabaseError, type Cast } from '../dist/index'
+import { connect, format, hex, DatabaseError, type Cast, Client, isClient } from '../dist/index'
 import { fetch, MockAgent, setGlobalDispatcher } from 'undici'
 import packageJSON from '../package.json'
 
@@ -616,3 +616,10 @@ describe('hex', () => {
     expect(hex('\0')).toEqual('0x00')
   })
 })
+
+describe('guards', () => {
+  test('isClient', () => {
+    const client = new Client(config)
+    expect(isClient(client)).toBe(true)
+  })
+})

src/index.ts

@@ -115,10 +115,17 @@ type ExecuteOptions<T extends ExecuteAs = 'object'> = T extends 'array'
   ? { as: 'array'; cast?: Cast }
   : never
 
+export const TypeId: unique symbol = Symbol.for('@planetscale/database/Client')
+export type TypeId = typeof TypeId
+
+export const isClient = (u: unknown): u is Client => typeof u === 'object' && u !== null && TypeId in u
+
 export class Client {
-  public readonly config: Config
+  public readonly config: Config;
+  readonly [TypeId]: TypeId
 
   constructor(config: Config) {
+    this[TypeId] = TypeId
     this.config = config
   }