A small utility for creating warnings and emitting them.

Downloads in past


1510.2.03 years ago3 years agoMinified + gzip package size for fastify-warning in KB


js-standard-style build
A small utility, used by Fastify itself, for generating consistent warning objects across your codebase and plugins. It also exposes a utility for emitting those warnings, guaranteeing that they are issued only once.


npm i fastify-warning


The module exports a builder function that returns an utility for creating warnings and emitting them.
const warning = require('fastify-warning')()


warning.create(name, code, message)

  • name (string, required) - The error name, you can access it later with For consistency, we recommend to prefix plugin error names with FastifWarning{YourPluginName}
  • code (string, required) - The warning code, you can access it later with error.code. For consistency, we recommend to prefix plugin error codes with FST_{YourPluginName}_
  • message (string, required) - The warning message. You can also use interpolated strings for formatting the message.

The utility also contains an emit function that you can use for emitting the warnings you have previously created by passing their respective code. A warning is guaranteed to be emitted only once.
warning.emit(code [, a [, b [, c]]])

  • code (string, required) - The warning code you intend to emit.
  • [, a [, b [, c]]] (any, optional) - Parameters for string interpolation.

const warning = require('fastify-warning')()
warning.create('FastifyWarning', 'FST_ERROR_CODE', 'message')

How to use a interpolated string:
const warning = require('fastify-warning')()
warning.create('FastifyWarning', 'FST_ERROR_CODE', 'Hello %s')
warning.emit('FST_ERROR_CODE', 'world')

The module also exports an warning.emitted Map, which contains all the warnings already emitted. Useful for testing.
const warning = require('fastify-warning')()
warning.create('FastifyWarning', 'FST_ERROR_CODE', 'Hello %s')
console.log(warning.emitted.get('FST_ERROR_CODE')) // false
warning.emit('FST_ERROR_CODE', 'world')
console.log(warning.emitted.get('FST_ERROR_CODE')) // true


Licensed under MIT.