Fix: Svelte Preprocessing Failing On .svelte.ts Files

by Alex Johnson 54 views

Svelte is a revolutionary approach to building user interfaces, shifting the bulk of the work from the browser to the compile step. This leads to incredibly performant web applications with minimal runtime overhead. One of the core features of Svelte is its preprocessing capabilities, allowing developers to integrate tools like TypeScript, SCSS, and CSS Modules seamlessly into their projects. However, a specific issue arises when working with Svelte 5 projects that use preprocess with .svelte.ts files. This article will explore the problem, the root cause, and how to resolve it, ensuring your Svelte projects compile correctly.

The Problem: Preprocessing and .svelte.ts Files

When using Svelte 5 with the esbuild-svelte plugin and enabling preprocessing, the expectation is that the preprocessors should only be applied to .svelte files. These are the files containing your Svelte components, where the framework's core logic resides. However, a bug in esbuild-svelte's handling of the preprocessor can cause it to mistakenly attempt to preprocess .svelte.ts files, which are meant to contain TypeScript code and not be processed by the Svelte preprocessors. This leads to compilation errors and a broken build.

Specifically, the error message often looks something like this, indicating the Svelte compiler is trying to parse TypeScript code as a Svelte component:

[redacted]:build: ✘ [ERROR] expected_token: Expected token }
[redacted]:build: https://svelte.dev/e/expected_token
[redacted]:build: 1: import { setContext as svelteSetContext } from "svelte";
[redacted]:build:                        ^
[redacted]:build: 2: export function createPlayerContext(player, setContext = svelteSetContext) {
[redacted]:build: 3:   const playerState = $state({});
[redacted]:build: 
[redacted]:build: The svelte component failed to be parsed. [plugin esbuild-svelte]
[redacted]:build: 
[redacted]:build:     src/index.ts:46:36:
[redacted]:build:       46 │ ...createPlayerContext } from './utils/create-player-context.svelte';
[redacted]:build:          ╵                               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This error arises because the Svelte compiler is expecting a Svelte component structure, but it encounters TypeScript code within the .svelte.ts file. This mismatch causes the compilation process to fail, preventing your project from building successfully. This is because the Svelte preprocessors (like TypeScript, SCSS, etc.) are designed to work on the .svelte files and not on .svelte.ts files. The intention is that .svelte.ts files are processed by the TypeScript compiler directly.

This behavior is a bug because the preprocessing step should be intelligently filtering out the .svelte.ts files from being processed by Svelte preprocessors, and only targeting the actual Svelte components. The current implementation, or the lack thereof, means it doesn't correctly differentiate between these two types of files, leading to the errors mentioned earlier.

The Root Cause: Lack of File Filtering

The root cause of this problem lies within the esbuild-svelte plugin itself. Specifically, the plugin's code for preprocessing doesn't include a filter to exclude .svelte.ts files. As a result, when preprocessing is enabled, the plugin attempts to preprocess all files matching the input, including these TypeScript files.

The existing implementation of the plugin doesn't have a mechanism to differentiate between .svelte and .svelte.ts files, leading to this incorrect preprocessing behavior. This oversight causes the TypeScript compiler to try to parse Svelte components, which results in the errors mentioned before. Without the correct file filtering, the preprocessors will be applied indiscriminately, causing compilation errors and preventing the project from building successfully.

The Solution: Implementing a File Filter

The good news is that this issue is easily fixed with a small patch to the esbuild-svelte plugin. The fix involves adding a filter to the preprocessing step to ensure that it only operates on .svelte files, excluding the .svelte.ts files. This ensures that the TypeScript files are handled correctly by the TypeScript compiler and not by the Svelte preprocessors.

Here's how you can implement this patch. The core of the solution is to check the file extension before applying the preprocessors. Specifically, modify the esbuild-svelte plugin to only apply preprocessing to .svelte files.

This code snippet shows the necessary change in the esbuild-svelte plugin. By adding the !SVELTE_TYPESCRIPT_MODULE_FILTER.test(filename) check, the plugin will now skip preprocessing for any file matching the pattern defined in the filter, which should correctly filter the .svelte.ts files:

diff --git a/node_modules/esbuild-svelte/dist/index.js b/node_modules/esbuild-svelte/dist/index.js
index b8584c5..2981bef 100644
--- a/node_modules/esbuild-svelte/dist/index.js
+++ b/node_modules/esbuild-svelte/dist/index.js
@@ -162,8 +162,9 @@ function sveltePlugin(options) {
         let moduleCompilerOptions = {
           ...options?.moduleCompilerOptions
         };
+
         try {
-          if (options?.preprocess) {
+          if (options?.preprocess && !SVELTE_TYPESCRIPT_MODULE_FILTER.test(filename)) {
             let preprocessResult = null;
             try {
               preprocessResult = await preprocess(source, options.preprocess, {

This patch introduces a conditional check (!SVELTE_TYPESCRIPT_MODULE_FILTER.test(filename)) before running the preprocessor. This ensures that the preprocessor is only applied to .svelte files and not .svelte.ts files, resolving the compilation error.

By applying this patch, you ensure that the preprocessors only run on the correct file types. This fixes the error and allows your project to build correctly, leveraging TypeScript and Svelte's powerful features.

Practical Implementation

There are several ways to apply this fix:

  1. Direct Patching (discouraged): You can directly modify the esbuild-svelte plugin files in your node_modules directory. However, this is not recommended because your changes will be overwritten every time you update the plugin. This approach is only useful for temporary testing and debugging.
  2. Patch-Package: The recommended method to maintain the patch is to use a tool like patch-package. This tool creates a patch file for the changes you make, which you can apply after each install or update of the plugin. This ensures your modifications persist across updates.
  3. Fork and Modify (advanced): Fork the esbuild-svelte repository on GitHub, apply the fix, and then use your forked version in your project. This approach gives you complete control but requires managing a separate repository and updating dependencies more manually.

The most practical solution is to use patch-package to apply this fix, allowing you to maintain the fix across updates while keeping your project clean.

Step-by-Step Guide to Implementing the Patch with patch-package

If you choose to use patch-package, follow these steps to apply the fix and ensure that it persists across updates:

  1. Install patch-package: In your project, install patch-package as a development dependency:
    npm install patch-package --save-dev
# or
yarn add patch-package --dev
  2. Apply the Patch: Make the changes to the esbuild-svelte plugin files (or if you already have, skip this step). Then, run the following command to create a patch file:
    npx patch-package esbuild-svelte
# or
yarn patch-package esbuild-svelte
    This command creates a patch file in your project's patches directory. The generated file describes the changes you made to the esbuild-svelte plugin.
  3. Integrate the Patch: Add the following script to your project's package.json file to ensure the patch is applied after each install:
    "scripts": {
  "postinstall": "patch-package"
}
    This ensures that the patch is automatically applied whenever you run npm install or yarn install.
  4. Test and Verify: Build your Svelte project to verify that the fix is applied and that your project compiles without errors. This will ensure that the fix has been applied correctly and is working as expected.

Following these steps, you will be able to apply the patch and solve the problem of preprocess not working on .svelte.ts files in your Svelte 5 projects.

Conclusion: Keeping Your Svelte Builds Clean

The issue of SVELTE_TYPESCRIPT_MODULE_FILTER not being applied in esbuild-svelte plugin is a crucial consideration for anyone working on Svelte 5 projects that use TypeScript and preprocessors. By understanding the root cause, and how to effectively apply the necessary patch, you can ensure that your Svelte projects build correctly, without the annoying errors that can halt your project. Remember to always keep your dependencies up to date, and use tools like patch-package to maintain any necessary patches. This ensures a clean and efficient build process, allowing you to focus on developing amazing Svelte applications.

External Links: