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# @fastify/error [](https://github.com/fastify/fastify-error/actions/workflows/ci.yml) [](https://www.npmjs.com/package/@fastify/error) [](https://github.com/neostandard/neostandard) A small utility, used by Fastify itself, for generating consistent error objects across your codebase and plugins. ### Install ``` npm i @fastify/error ``` ### Usage The module exports a function that you can use for consistent error objects, it takes 4 parameters: ```js createError(code, message [, statusCode [, Base [, captureStackTrace]]]) ``` - `code` (`string`, required) - The error code, you can access it later with `error.code`. For consistency, we recommend prefixing plugin error codes with `FST_` - `message` (`string`, required) - The error message. You can also use interpolated strings for formatting the message. - `statusCode` (`number`, optional) - The status code that Fastify will use if the error is sent via HTTP. - `Base` (`ErrorConstructor`, optional) - The base error object that will be used. (eg `TypeError`, `RangeError`) - `captureStackTrace` (`boolean`, optional) - Whether to capture the stack trace or not. ```js const createError = require('@fastify/error') const CustomError = createError('ERROR_CODE', 'Hello') console.log(new CustomError()) // error.message => 'Hello' ``` How to use an interpolated string: ```js const createError = require('@fastify/error') const CustomError = createError('ERROR_CODE', 'Hello %s') console.log(new CustomError('world')) // error.message => 'Hello world' ``` How to add cause: ```js const createError = require('@fastify/error') const CustomError = createError('ERROR_CODE', 'Hello %s') console.log(new CustomError('world', {cause: new Error('cause')})) // error.message => 'Hello world' // error.cause => Error('cause') ``` ### TypeScript It is possible to limit your error constructor with a generic type using TypeScript: ```ts const CustomError = createError<[string]>('ERROR_CODE', 'Hello %s') new CustomError('world') //@ts-expect-error new CustomError(1) ``` ### instanceof All errors created with `createError` will be instances of the base error constructor you provided, or `Error` if none was provided. ```js const createError = require('@fastify/error') const CustomError = createError('ERROR_CODE', 'Hello %s', 500, TypeError) const customError = new CustomError('world') console.log(customError instanceof CustomError) // true console.log(customError instanceof TypeError) // true console.log(customError instanceof Error) // true ``` All instantiated errors are instances of the `FastifyError` class, which can be required directly from the module. ```js const { createError, FastifyError } = require('@fastify/error') const CustomError = createError('ERROR_CODE', 'Hello %s', 500, TypeError) const customError = new CustomError('world') console.log(customError instanceof FastifyError) // true ``` A `FastifyError` created by `createError` can extend another `FastifyError` while maintaining correct `instanceof` behavior. ```js const { createError, FastifyError } = require('@fastify/error') const CustomError = createError('ERROR_CODE', 'Hello %s', 500, TypeError) const ChildCustomError = createError('CHILD_ERROR_CODE', 'Hello %s', 500, CustomError) const customError = new ChildCustomError('world') console.log(customError instanceof ChildCustomError) // true console.log(customError instanceof CustomError) // true console.log(customError instanceof FastifyError) // true console.log(customError instanceof TypeError) // true console.log(customError instanceof Error) // true ``` If `fastify-error` is installed multiple times directly or as a transitive dependency, `instanceof` checks for errors created by `createError` will still work correctly across these installations, as long as their error codes (e.g., `FST_ERR_CUSTOM_ERROR`) are identical. ```js const { createError, FastifyError } = require('@fastify/error') // CustomError from `@fastify/some-plugin` is created with `createError` and // has its own `@fastify/error` installation as dependency. CustomError has // FST_ERR_CUSTOM_ERROR as code. const { CustomError: CustomErrorFromPlugin } = require('@fastify/some-plugin') const CustomError = createError('FST_ERR_CUSTOM_ERROR', 'Hello %s', 500) const customError = new CustomError('world') const customErrorFromPlugin = new CustomErrorFromPlugin('world') console.log(customError instanceof CustomError) // true console.log(customError instanceof CustomErrorFromPlugin) // true console.log(customErrorFromPlugin instanceof CustomError) // true console.log(customErrorFromPlugin instanceof CustomErrorFromPlugin) // true ``` Changing the code of an instantiated Error will not change the result of the `instanceof` operator. ```js const { createError, FastifyError } = require('@fastify/error') const CustomError = createError('ERROR_CODE', 'Hello %s', 500, TypeError) const AnotherCustomError = createError('ANOTHER_ERROR_CODE', 'Hello %s', 500, CustomError) const customError = new CustomError('world') customError.code = 'ANOTHER_ERROR_CODE' console.log(customError instanceof CustomError) // true console.log(customError instanceof AnotherCustomError) // false ``` ## License Licensed under [MIT](./LICENSE).