Fix inaccurate type of setErrorHandler (#2092)

* fix: type of `setErrorHandler`

* fix: use generic instead of any

* add test for the type of `setErrorHandler`

fastify.d.ts

@@ -693,7 +693,7 @@ declare namespace fastify {
     /**
      * Set a function that will be called whenever an error happens
      */
-    setErrorHandler(handler: (error: FastifyError, request: FastifyRequest<HttpRequest>, reply: FastifyReply<HttpResponse>) => void): void
+    setErrorHandler<E = FastifyError>(handler: (error: E, request: FastifyRequest<HttpRequest>, reply: FastifyReply<HttpResponse>) => void): void
 
     /**
      * Set a function that will be called whenever an error happens

test/types/index.ts

@@ -519,6 +519,30 @@ server.setErrorHandler((err, request, reply) => {
   }
 })
 
+// Libraries may define their own error types
+class AuthenticationError extends Error {
+  public constructor (public data: string) {
+    super()
+  }
+}
+
+class DatabaseError extends Error {
+  public constructor (public query: string) {
+    super()
+  }
+}
+
+// Users can deal with those errors via error handlers
+server.setErrorHandler<AuthenticationError | DatabaseError>((err, request, reply) => {
+  if (err instanceof AuthenticationError) {
+    server.log.error(err.data)
+    reply.send(err.message)
+  } else {
+    server.log.error(err.query)
+    reply.send(err.message)
+  }
+})
+
 server.setReplySerializer((payload, statusCode) => {
   if (statusCode === 201) {
     return `Created ${payload}`