fastify/docs/Reference/Warnings.md

Fast and low overhead web framework, for Node.js

<h1 align="center">Fastify</h1>

**Table of contents**
- [Warnings](#warnings)
  - [Warnings In Fastify](#warnings-in-fastify)
  - [Fastify Warning Codes](#fastify-warning-codes)
    - [FSTWRN001](#FSTWRN001)
    - [FSTWRN003](#FSTWRN003)
    - [FSTWRN004](#FSTWRN004)
  - [Fastify Deprecation Codes](#fastify-deprecation-codes)
    - [FSTDEP022](#FSTDEP022)
    - [FSTDEP023](#FSTDEP023)
    - [FSTDEP024](#FSTDEP024)

## Warnings

### Warnings In Fastify

Fastify uses the Node.js [warning event](https://nodejs.org/api/process.html#event-warning)
API to notify users of deprecated features and coding mistakes. Fastify's
warnings are recognizable by the `FSTWRN` and `FSTDEP` prefixes. When
encountering such a warning, it is highly recommended to determine the cause
using the [`--trace-warnings`](https://nodejs.org/api/cli.html#trace-warnings)
and [`--trace-deprecation`](https://nodejs.org/api/cli.html#trace-deprecation)
flags. These produce stack traces pointing to where the issue occurs in the
application's code. Issues opened about warnings without this information will
be closed.

Warnings can also be disabled, though it is not recommended. If necessary, use
one of the following methods:

- Set the `NODE_NO_WARNINGS` environment variable to `1`
- Pass the `--no-warnings` flag to the node process
- Set `no-warnings` in the `NODE_OPTIONS` environment variable

For more information on disabling warnings, see [Node's documentation](https://nodejs.org/api/cli.html).

Disabling warnings is not recommended and may cause unexpected behavior.

### Fastify Warning Codes

| Code | Description | How to solve | Discussion |
| ---- | ----------- | ------------ | ---------- |
| <a id="FSTWRN001">FSTWRN001</a> | The specified schema for a route is missing. This may indicate the schema is not well specified. | Check the schema for the route. | [#4647](https://github.com/fastify/fastify/pull/4647) |
| <a id="FSTWRN003">FSTWRN003</a> | The `%s` plugin mixes async and callback styles, which may lead to unhandled rejections. | Do not mix async and callback style. | [#6011](https://github.com/fastify/fastify/pull/6011) |
| <a id="FSTWRN004">FSTWRN004</a> | An `errorHandler` is being overridden in the same scope, which can lead to subtle bugs. | Avoid calling `setErrorHandler` more than once in the same scope. For more information, see [Server documentation](https://fastify.dev/docs/latest/Reference/Server/#allowerrorhandleroverride). | [#6104](https://github.com/fastify/fastify/pull/6104) |


### Fastify Deprecation Codes

Deprecation codes are supported by the Node.js CLI options:

- [--no-deprecation](https://nodejs.org/api/cli.html#no-deprecation)
- [--throw-deprecation](https://nodejs.org/api/cli.html#throw-deprecation)
- [--trace-deprecation](https://nodejs.org/api/cli.html#trace-deprecation)


| Code | Description | How to solve | Discussion |
| ---- | ----------- | ------------ | ---------- |
| <a id="FSTDEP022">FSTDEP022</a> | You are trying to access the deprecated router options on top option properties. | Use `options.routerOptions`. | [#5985](https://github.com/fastify/fastify/pull/5985)
| <a id="FSTDEP023">FSTDEP023</a> | `disableRequestLogging` top-level option is deprecated. | Pass a `LogController` instance via the `logController` option with `disableRequestLogging` in its constructor instead. |
| <a id="FSTDEP024">FSTDEP024</a> | `requestIdLogLabel` top-level option is deprecated. | Pass a `LogController` instance via the `logController` option with `requestIdLogLabel` in its constructor instead. |