fix: stricter types for unsigned cookies (#292)

The success and failure cases are distinct and the types can reflect
this.

Author: ojeytonwilliams

types/plugin.d.ts

@@ -155,10 +155,14 @@ declare namespace fastifyCookie {
   export type Unsign = (input: string, secret: string | Buffer, algorithm?: string) => UnsignResult;
   export type SignerFactory = (secrets: string | string[] | Buffer | Buffer[], algorithm?: string) => SignerBase;
 
-  export interface UnsignResult {
-    valid: boolean;
+  export type UnsignResult = {
+    valid: true;
     renew: boolean;
-    value: string | null;
+    value: string;
+  } | {
+    valid: false;
+    renew: false;
+    value: null;
   }
 
   export const signerFactory: SignerFactory;

types/plugin.test-d.ts

@@ -150,11 +150,15 @@ appWithRotationSecret.register(cookie, {
 appWithRotationSecret.after(() => {
   server.get('/', (request, reply) => {
     reply.unsignCookie(request.cookies.test!);
-    const { valid, renew, value } = reply.unsignCookie('test');
+    const unsigned = reply.unsignCookie('test');
 
-    expectType<boolean>(valid);
-    expectType<boolean>(renew);
-    expectType<string | null>(value);
+    expectType<boolean>(unsigned.valid);
+    if (unsigned.valid) {
+      expectType<string>(unsigned.value);
+    } else {
+      expectType<null>(unsigned.value);
+    }
+    expectType<boolean>(unsigned.renew);
 
     reply.send({ hello: 'world' });
   });
@@ -182,11 +186,15 @@ appWithParseOptions.register(cookie, {
 });
 appWithParseOptions.after(() => {
   server.get('/', (request, reply) => {
-    const { valid, renew, value } = reply.unsignCookie(request.cookies.test!);
+    const unsigned = reply.unsignCookie(request.cookies.test!);
 
-    expectType<boolean>(valid);
-    expectType<boolean>(renew);
-    expectType<string | null>(value);
+    expectType<boolean>(unsigned.valid);
+    if (unsigned.valid) {
+      expectType<string>(unsigned.value);
+    } else {
+      expectType<null>(unsigned.value);
+    }
+    expectType<boolean>(unsigned.renew);
   });
 });
 
@@ -204,11 +212,15 @@ appWithCustomSigner.register(cookie, {
 appWithCustomSigner.after(() => {
   server.get('/', (request, reply) => {
     reply.unsignCookie(request.cookies.test!)
-    const { valid, renew, value } = reply.unsignCookie('test')
+    const unsigned = reply.unsignCookie('test')
 
-    expectType<boolean>(valid)
-    expectType<boolean>(renew)
-    expectType<string | null>(value)
+    expectType<boolean>(unsigned.valid);
+    if (unsigned.valid) {
+      expectType<string>(unsigned.value);
+    } else {
+      expectType<null>(unsigned.value);
+    }
+    expectType<boolean>(unsigned.renew);
 
     reply.send({ hello: 'world' })
   })