Fastify reply was can not be sent - nest

Question

export class SessionController {
constructor(private sessionService: SessionService) {}
#Post('login')
async login(
#Req() req: FastifyRequest,
#Body() params: LoginDto,
#Res() reply: FastifyReply,
) {
params.ipAddress = req.socket.remoteAddress; /* ::1 */
this.sessionService
.signin(params)
.then((user) => {
reply.header('Authorization', `Bearer ${user.access_token}`);
reply.code(201).send(user);
})
.catch((error) => {
console.log(error instanceof HttpException);
reply.status(error.status).send({ errors: [error.message] });
});
}
}
When I hit this API, I received an error which was reply was already sent, Here I did not use the reply.sent() function two times, In this nest js application I have used the frame framework under the hood, If I run this same function with express it works fine.
{"level":40,"time":1663844608468,"pid":5951,"hostname":"yavar","reqId":"req-1","err":{"type":"FastifyError","message":"Reply was already sent.","stack":"FastifyError: Reply was already sent.\n at Reply.send (/home/bennison/Documents/project/nest-lib/node_modules/fastify/lib/reply.js:118:26)\n at /home/bennison/Documents/project/nest-lib/src/user/user.controller.ts:35:15\n at processTicksAndRejections (node:internal/process/task_queues:96:5)","name":"FastifyError","code":"FST_ERR_REP_ALREADY_SENT","statusCode":500},"msg":"Reply already sent"}
If I use the async and await method instead of then catch it works fine. If anyone knows the answer, please explain to me what mistake have I made here.

Answer 1

To fix that issue, we need to call the method in the return statement. here call the sessionService in the return statement.
export class SessionController {
constructor(private sessionService: SessionService) {}
#Post('login')
async login(
#Req() req: FastifyRequest,
#Body() params: LoginDto,
#Res() reply: FastifyReply,
) {
params.ipAddress = req.socket.remoteAddress; /* ::1 */
return this.sessionService
.signin(params)
.then((user) => {
reply.header('Authorization', `Bearer ${user.access_token}`);
reply.code(201).send(user);
})
.catch((error) => {
console.log(error instanceof HttpException);
reply.status(error.status).send({ errors: [error.message] });
});
}
}