What's the problem?

When using outExtension: () => ({ js: ".jsx" }) Tsup doesn not generate source maps. The build works but the missing source maps lead to a very bad debugging experience for us. Would be great to have these for jsx outputs as well.

Longer Story

We're using SolidJS and need to preserve the JSX in library code of a big monorepo. The libs are built with tsup while the application is built with vite. The application uses vite-plugin-solid which expects all library code expose the unbuilt jsx and then builds the JSX in files with .jsx extensions in one batch. The build works but the missing source maps lead to a very bad debugging experience for us.

Reproduction

// tsup.config.ts
import { defineConfig } from "tsup";

export default defineConfig({
  format: "esm",
  entry: ["src/index.tsx"],
  outDir: "out",
  sourcemap: true,
  esbuildOptions(es_options) {
    es_options.jsx = "preserve";
  },
  outExtension: () => ({ js: ".jsx" }),
});

// src/index.tsx
export function Component() {
  return <div style="color: red; font-weight: bold;">Hello</div>;
}

// package.json
{
  "name": "lib",
  "type": "module",
  "exports": "./out/index.jsx",
  "scripts": {
    "build": "tsup"
  },
  "dependencies": {
    "solid-js": "~1.9.4"
  },
  "devDependencies": {
    "typescript": "~5.2.2",
    "tsup": "~8.3.6"
  }
}

Now run tsup

Expected behavior

The out folder should contain index.jsx and index.jsx.map.

Actual behavior

The out folder only contains index.jsx and no index.jsx.map.

Narrowing it down

When commenting out the outExtension option in the tsup.config.ts the maps start appearing.

Additional info

It does not seem to be an upstream esbuild problem. I ran esbuild with the same config that tsup calls it with and the source maps were there. They aren't when run through tsup though. I think the issue might lie in the this code that's responsible for writing to disc