Logging Fastify response body - node.js

Question

How can I log the response body in Fastify? The body doesn't seem to be exposed as part of the response object:
const fastify = require('fastify')({
logger: {
serializers: {
res: function (res) {
// No body in req afaik
return { }
}
}
}
})

Answer 1

Try this:
const fastify = require('fastify')({
logger: {
serializers: {
res: function (res) {
return {
statusCode: res.statusCode,
payload: res.payload,
}
},
}
}
})
fastify.addHook('onSend', function (_request, reply, payload, next) {
Object.assign(reply.res, { payload });
next();
})
If some of your payloads are objects and you want to get them in serialize before they are - well, serialized - you can add preSerialization hook as well:
fastify
.addHook('preSerialization', (_request, reply, payload, next) => {
Object.assign(reply.res, { payload });
next();
})
.addHook('onSend', (_request, reply, payload, next) => {
if (!reply.res.payload) Object.assign(reply.res, { payload });
next();
});

Answer 2

here you are a working example. I think that this kind of usage need to be used only for debugging because you are slowing down if you have many req/sec.
I have also added a JSON Schema validation as demo:
const fastify = require('fastify')({ logger: true })
fastify.register(async function (fastify, opts) {
fastify.addHook('onSend', function (request, reply, payload, next) {
console.log(payload);
next()
})
fastify.get('/', {
schema: {
response: {
'2xx': { properties: { this: { type: 'string' } } }
}
}
}, async function () {
return { this: 'is', a: 'json' }
})
})
fastify.listen(3000)
You will get:
curl http://localhost:3000/
{"this":"is"}