Could someone help me to create a HOOK in FASTIFY to automatically serialize these types of values ​in all app.ts endpoints?

Question

Could someone help me to create a HOOK in FASTIFY to automatically serialize these types of values ​​in all app.ts endpoints

I managed to solve the problem using .toString, but I wanted a way to do this automatically in all requests

Answer 1

First of all, bigint is not supported by JS standard functions:

const big = { n: :1231231231231231213n }
console.log(typeof big.n) // bigint
JSON.stringify(big) // TypeError: Do not know how to serialize a BigInt

JSON.parse('{"n":1231231231231231213}') // { n: 1231231231231231200 }

Here is 3 solutions:

1. set a json schema response and fastify will do it for you by converting them into numbers!
2. take ownership of the serialization with preSerialization hook. Note that the toString returns a string a not a number
3. take ownership of the serialization with setReplySerializer. Different approach but same result as before.

I would use something that gives fully control over the serialization, because serializing bigint is not supported by the JSON.* utilities and the client who parse the json must be aware of it or it will fail! See the code snippet above, JSON.parse() trims the number.

const app = require('fastify')({ logger: true })

app.get('/solution-1', {
  schema: {
    response: {
      200: {
        type: 'object',
        properties: {
          n: { type: 'number' }
        }
      }
    }
  }
}, async (request, reply) => {
  return { n: 1231231231231231213n } // prints {"n":1231231231231231200} (not a string 🚀)
})

app.register(function plugin (app, opts, next) {
  app.addHook('preSerialization', async (request, reply, payload) => {
    payload.n = payload.n.toString()
    return payload
  })

  app.get('/solution-2', async (request, reply) => {
    return { n: 1231231231231231213n } // prints {"n":"1231231231231231213"}
  })

  next()
})

app.register(function plugin (app, opts, next) {
  app.setReplySerializer(function (payload, statusCode) {
    return JSON.stringify(payload, (key, value) =>
      typeof value === 'bigint'
        ? value.toString()
        : value
    )
  })
  app.get('/solution-3', async (request, reply) => {
    return { n: 1231231231231231213n } // prints {"n":"1231231231231231213"}
  })

  next()
})

app.listen({ port: 8080 })

curl http://localhost:8080/solution-1
curl http://localhost:8080/solution-2
curl http://localhost:8080/solution-3