Draft: Add error handler package

packages/error-handler/.eslintignore

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+.eslintrc.cjs
+coverage
+dist
+node_modules

packages/error-handler/.eslintrc.cjs

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+module.exports = {
+  root: true,
+  extends: ["@prefabs.tech/eslint-config/fastify"],
+};

packages/error-handler/.gitignore

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+**/*.log*
+/coverage
+/dist
+/node_modules

packages/error-handler/README.md

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+# @prefabs.tech/fastify-error-handler
+
+A [Fastify](https://github.com/fastify/fastify) plugin that provides an easy integration of error handler in fastify API.
+
+## Requirements
+
+* [@prefabs.tech/fastify-config](../config/)
+* [@fastify/sensible](https://github.com/fastify/fastify-sensible)
+
+## Installation
+
+Install with npm:
+
+```bash
+npm install @prefabs.tech/fastify-error-handler
+```
+
+Install with pnpm:
+
+```bash
+pnpm add --filter "@scope/project @prefabs.tech/fastify-error-handler
+```
+
+## Usage
+
+### Register Plugin
+
+Register @prefabs.tech/fastify-error-handler package with your Fastify instance:
+
+Note: Register the errorHandler plugin as early as possible (Before all your routes and plugin registration).
+
+```typescript
+import errorHandlerPlugin from "@prefabs.tech/fastify-error-handler";
+import Fastify from "fastify";
+
+const start = async () => {
+  // Create fastify instance
+  const fastify = Fastify();
+
+  // Register fastify-error-handler plugin
+  await fastify.register(errorHandlerPlugin, {});
+
+  await fastify.listen({
+    port: config.port,
+    host: "0.0.0.0",
+  });
+};
+
+start();
+```
+### Options
+
+#### stackTrace
+
+When enabled, the error handler will include the error’s stack trace in the HTTP response body.
+
+By default, it is set to false.
+
+```ts
+stackTrace?: boolean; // Default: false
+```
+
+#### preErrorHandler
+
+preErrorHandler is an optional error handler that runs before the default error handler logic.
+It allows you to intercept specific errors, handle them yourself, and prevent the default handler from running.
+
+This is especially useful when you need to integrate with other libraries that have their own error formats — for example, handling SuperTokens errors before your API’s standard error response.
+
+```ts
+preErrorHandler?: (
+  error: FastifyError,
+  request: FastifyRequest,
+  reply: FastifyReply,
+) => void | Promise<void>;
+```

packages/error-handler/package.json

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+{
+  "name": "@prefabs.tech/fastify-error-handler",
+  "version": "0.88.2",
+  "description": "Fastify error-handler plugin",
+  "homepage": "https://github.com/prefabs-tech/fastify/tree/main/packages/error-handler#readme",
+  "repository": {
+    "type": "git",
+    "url": "git+https://github.com/prefabs-tech/fastify.git",
+    "directory": "packages/error-handler"
+  },
+  "license": "MIT",
+  "type": "module",
+  "exports": {
+    ".": {
+      "import": "./dist/prefabs-tech-fastify-error-handler.js",
+      "require": "./dist/prefabs-tech-fastify-error-handler.umd.cjs"
+    }
+  },
+  "main": "./dist/prefabs-tech-fastify-error-handler.umd.cjs",
+  "module": "./dist/prefabs-tech-fastify-error-handler.js",
+  "types": "./dist/types/index.d.ts",
+  "files": [
+    "dist"
+  ],
+  "scripts": {
+    "build": "vite build && tsc --emitDeclarationOnly && mv dist/src dist/types",
+    "lint": "eslint .",
+    "lint:fix": "eslint . --fix",
+    "sort-package": "npx sort-package-json",
+    "typecheck": "tsc --noEmit -p tsconfig.json --composite false"
+  },
+  "dependencies": {
+    "@fastify/sensible": "6.0.3",
+    "stack-trace":"1.0.0-pre2"
+  },
+  "devDependencies": {
+    "@prefabs.tech/eslint-config": "0.2.0",
+    "@prefabs.tech/tsconfig": "0.2.0",
+    "@types/node": "20.19.9",
+    "@types/stack-trace": "0.0.33",
+    "@typescript-eslint/eslint-plugin": "8.38.0",
+    "@typescript-eslint/parser": "8.38.0",
+    "@vitest/coverage-istanbul": "3.2.4",
+    "eslint": "8.57.1",
+    "eslint-config-prettier": "9.1.2",
+    "eslint-import-resolver-alias": "1.1.2",
+    "eslint-import-resolver-typescript": "3.10.1",
+    "eslint-plugin-import": "2.32.0",
+    "eslint-plugin-n": "14.0.0",
+    "eslint-plugin-prettier": "5.5.3",
+    "eslint-plugin-promise": "7.2.1",
+    "eslint-plugin-unicorn": "56.0.1",
+    "fastify": "5.4.0",
+    "fastify-plugin": "5.0.1",
+    "prettier": "3.6.2",
+    "typescript": "5.8.3",
+    "vite": "6.3.5",
+    "vitest": "3.2.4"
+  },
+  "peerDependencies": {
+    "fastify": ">=5.2.1",
+    "fastify-plugin": ">=5.0.1"
+  },
+  "engines": {
+    "node": ">=20"
+  }
+}

packages/error-handler/src/errorHandler.ts

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+import { STATUS_CODES } from "node:http";
+
+import { HttpError } from "@fastify/sensible";
+import { FastifyReply, FastifyRequest } from "fastify";
+import { parse } from "stack-trace";
+
+import { CustomError } from "./utils/error";
+
+import type { ErrorResponse } from "./types";
+
+const getHttpStatusText = (statusCode: number): string =>
+  STATUS_CODES[statusCode] ?? "Internal Server Error";
+
+export const errorHandler = (
+  error: Error,
+  request: FastifyRequest,
+  reply: FastifyReply,
+) => {
+  const { log: logger } = request;
+
+  const isStackTraceEnabled = request.server.stackTrace || false;
+
+  const isHttpError = error instanceof HttpError;
+
+  if (isHttpError) {
+    const statusCode = error.statusCode || 500;
+
+    if (statusCode >= 500) {
+      logger.error(error);
+    } else if (statusCode >= 400) {
+      logger.info(error);
+    } else {
+      logger.error(error);
+    }
+
+    const response: ErrorResponse = {
+      code: error.code,
+      error: error.error || getHttpStatusText(statusCode),
+      message: error.message,
+      name: error.name,
+      statusCode,
+    };
+
+    if (isStackTraceEnabled && error.stack) {
+      response.stack = parse(error);
+    }
+
+    void reply.code(statusCode).send(response);
+
+    return;
+  }
+
+  let message = "Server error, please contact support";
+  let code = "INTERNAL_SERVER_ERROR";
+
+  if (error instanceof CustomError) {
+    code = error.code || code;
+    message = "Server has an error that is not handled, please contact support";
+  }
+
+  if (isStackTraceEnabled && error.stack) {
+    const response: ErrorResponse = {
+      code: code,
+      message: error.message,
+      name: error.name,
+      statusCode: 500,
+    };
+
+    response.stack = parse(error);
+
+    void reply.code(500).send(response);
+
+    return;
+  }
+
+  // remove stack and message from error
+  delete error.stack;
+  error.message = message;
+
+  // let fastify handle the error
+  throw error;
+};

packages/error-handler/src/index.ts

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+import type { HttpErrors } from "@fastify/sensible";
+
+declare module "fastify" {
+  interface FastifyInstance {
+    httpErrors: HttpErrors;
+    stackTrace: boolean;
+  }
+}
+
+export { default } from "./plugin";
+
+export { errorHandler } from "./errorHandler";
+
+export { CustomError } from "./utils/error";
+
+export type { HttpErrors } from "@fastify/sensible";
+
+export type * from "./types";

packages/error-handler/src/plugin.ts

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+import fastifySensible from "@fastify/sensible";
+import FastifyPlugin from "fastify-plugin";
+
+import { errorHandler } from "./errorHandler";
+
+import type { ErrorHandlerOptions } from "./types";
+import type { FastifyInstance } from "fastify";
+
+const plugin = async (
+  fastify: FastifyInstance,
+  options: ErrorHandlerOptions,
+) => {
+  fastify.log.info("Registering fastify-error-handler plugin");
+
+  fastify.decorate("stackTrace", options.stackTrace || false);
+
+  await fastify.register(fastifySensible);
+
+  fastify.setErrorHandler(async (error, request, reply) => {
+    if (options.preErrorHandler) {
+      try {
+        await options.preErrorHandler(error, request, reply);
+      } catch {
+        // If preErrorHandler throws an error, we can ignore it and continue
+      }
+
+      if (reply.sent) {
+        return;
+      }
+    }
+
+    return errorHandler(error, request, reply);
+  });
+};
+
+export default FastifyPlugin(plugin);

packages/error-handler/src/types.ts

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+import { FastifyError, FastifyRequest, FastifyReply } from "fastify";
+
+import type { StackFrame } from "stack-trace";
+
+type ErrorHandler = (
+  error: FastifyError,
+  request: FastifyRequest,
+  reply: FastifyReply,
+) => void | Promise<void>;
+
+interface ErrorHandlerOptions {
+  preErrorHandler?: ErrorHandler;
+  stackTrace?: boolean;
+}
+
+type ErrorResponse = {
+  error?: string;
+  code?: string;
+  message: string;
+  name: string;
+  stack?: StackFrame[];
+  statusCode: number;
+};
+
+export type { ErrorHandler, ErrorHandlerOptions, ErrorResponse };
+
+export { type StackFrame } from "stack-trace";

packages/error-handler/src/utils/error.ts

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+export class CustomError extends Error {
+  public code?: string;
+
+  constructor(message: string, code?: string) {
+    super(message);
+
+    this.code = code;
+    this.name = this.constructor.name; // sets name to "CustomError" so that it works in logs
+
+    // (error instanceof CustomError) and (error instanceof Error) both works because of this
+    Object.setPrototypeOf(this, new.target.prototype);
+  }
+}

packages/error-handler/tsconfig.json

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+{
+  "extends": "@prefabs.tech/tsconfig/fastify.json",
+  "compilerOptions": {
+    "outDir": "./dist",
+  },
+  "include": [
+    "src/**/*.ts"
+  ]
+}