Fixing Vercel API Deployments: ERR_MODULE_NOT_FOUND

by Admin 52 views
Fixing Vercel API Deployments: ERR_MODULE_NOT_FOUND

Hey guys, have you ever run into a frustrating issue when deploying your API to Vercel? You get everything set up, the deployment seems successful, but then BAM! You're hit with an ERR_MODULE_NOT_FOUND error, specifically related to your path aliases. If you've been there, you're definitely not alone. This guide is all about helping you understand and fix the @api/* path alias issue when deploying your Hono API on Vercel. We'll dive deep into the problem, why it happens, and most importantly, how to solve it. Let's get started, shall we?

The Problem: @api/* Path Aliases Fail on Vercel

So, what exactly is going wrong? Let's break it down. You're building a cool API, probably using TypeScript to keep things organized. You're using path aliases like @api/features to neatly reference different parts of your project. This is a super common and effective way to manage your codebase, making it cleaner and easier to navigate. You deploy your app to Vercel, everything appears to be deploying okay, but when the application tries to run, it hits a roadblock. The server can't find these @api/* paths, and throws an ERR_MODULE_NOT_FOUND error. This means the server can’t find the modules you’re trying to import using your path aliases.

Understanding the Error

The core of the problem lies in how Vercel's zero-configuration deployment, when used with something like the @vercel/hono builder, handles your code. When you use path aliases, your TypeScript compiler knows how to resolve them during development and build time. However, Vercel’s deployment process doesn't inherently bundle or transform your code in a way that understands these aliases at runtime. When your server tries to run, it looks for the modules in the way Node.js would, which, by default, is to look for npm packages. Since your @api/* paths aren't actual npm packages, the server can't find them, causing the ERR_MODULE_NOT_FOUND error.

Reproduction Steps & Symptoms

Let’s summarize the steps that cause this issue and what you’ll see when it happens.

  1. Deployment to Vercel: Deploy your API application, typically located in a directory like apps/api, to Vercel using the zero-config Hono deployment. Vercel automatically detects the Hono app and uses the @vercel/hono builder.
  2. Successful Deployment, Initial Failure: The deployment process itself appears successful. Vercel indicates that the application has been deployed.
  3. Runtime Errors: However, when the application attempts to execute, it fails. Check Vercel function logs, and you will see the ERR_MODULE_NOT_FOUND error. For example: Error [ERR_MODULE_NOT_FOUND]: Cannot find package '@api/features' imported from /var/task/apps/api/src/index.js.

The Root Cause: Path Aliases and Bundling

To understand the solution, you need to understand the root cause. Your tsconfig.json file is a crucial player here. If your tsconfig.json uses moduleResolution: "bundler", it expects a bundler (like Webpack, Parcel, or esbuild) to resolve the path aliases. But in the case of Vercel’s zero-config deployment, there's no bundling step happening by default. Your apps/api/tsconfig.json likely overrides this to moduleResolution: "Node", and while path aliases work during compile time, they fail during the Node.js runtime because they aren't bundled.

Solutions: Resolving the Path Alias Issue

Alright, now that we understand the problem, let's explore the solutions. There are a few approaches you can take to resolve the @api/* path alias issue and get your API up and running smoothly on Vercel. We will prioritize the best solution: using a bundler.

Option 1: Add tsup/esbuild Bundling (Recommended)

This is the most effective and recommended solution, and likely the best for maintainability and performance. The goal here is to bundle your API code before deploying it to Vercel. This means taking all your code and dependencies, resolving the path aliases, and packaging it into a single, deployable unit. This approach is highly effective because it ensures that all of your paths are correctly resolved at build time.

  • How it Works: Tools like tsup and esbuild are incredibly efficient bundlers. They take your TypeScript code, resolve all the imports (including those using path aliases), and output a single JavaScript file or a set of files that Vercel can run. This means that when Vercel runs your code, it doesn't need to worry about resolving path aliases because everything is already resolved.
  • Implementation:

    1. Install a Bundler: First, install the bundler of your choice (tsup is used as example): npm install tsup --save-dev.

    2. Configure tsup: Create a tsup.config.ts file in your apps/api directory.

    3. Example tsup.config.ts:

      import { defineConfig } from 'tsup';

export default defineConfig({
  entry: ['src/index.ts'], // or your entry point
  splitting: false, // optional, for performance
  sourcemap: true,
  clean: true,
  // Add any other bundling options here
});

    4. Update your build script: Modify your package.json to use tsup to build your API before deployment. For example, add a build script like `