Customizing the Error Output

Introduction

In the 5.34.0 version we added a possibility for users to modify our Fastify handler instance, via the ModifyFastifyPlugin. To find our more on how and why we use Fastify, please read this article.

With the given plugin user can change and add anything to the Fastify instance, including the error handler.

We have our built-in error handler, but if it does not suit the user, they can change it.

Creating the Modifier Plugin to Set a Custom Error Handler

There is a plugin class, ModifyFastifyPlugin, which user can construct, or they can use a function which in turn constructs the plugin from the ModifyFastifyPlugin class.

Example: Create a Custom Handler via Class

import { ModifyFastifyPlugin } from "@webiny/handler";

const plugin = new ModifyFastifyPlugin(instance => {
  instance.setErrorHandler(async (error, request, reply) => {
    return reply
      .status(500)
      .headers({
        "Cache-Control": "no-store"
      })
      .send({
        ...error,
        myCustomErrorClassHandler: true
      });
  });
});

Example: Create a Custom Handler via Function

import { createModifyFastifyPlugin } from "@webiny/handler";

const plugin = createModifyFastifyPlugin(instance => {
  instance.setErrorHandler(async (error, request, reply) => {
    return reply
      .status(500)
      .headers({
        "Cache-Control": "no-store"
      })
      .send({
        ...error,
        myCustomErrorFunctionHandler: true
      });
  });
});

Registering the Plugin

In both cases of creating the Fastify modifier plugin, users need to register the plugin the same location.

export const handler = createHandler({
  plugins: [
    // ... register the plugin in the plugins array of the createHandler
    plugin
  ]
});