TypeScript type-only export specifiers cause esbuild to overwrite module.exports in esbuild >0.14.3

[p0wl]

Since esbuild 0.14.4 (0.14.5) a type-only export causes esbuild to apply the __toCommonJS() modifier to module.exports. this causes all other properties on module.exports to be overwritten.

I see that using export causes esbuild to apply the CJS transformation, but we were running into troubles because we use type only exports and those were removed without overwriting module.exports before v0.14.4.

Here is what happens with 0.14.3:

➜ npx esbuild --version; echo 'export type Area = 'ASSEMBLY' | 'BERLIN'; module.exports.bla = { a: 1 }' | npx esbuild --format=cjs --loader=ts
0.14.3
var __defProp = Object.defineProperty;
var __markAsModule = (target) => __defProp(target, "__esModule", { value: true });
__markAsModule(exports);
module.exports.bla = { a: 1 };

and since 0.14.5 the behavior changed:

0.14.5
var __defProp = Object.defineProperty;
...
var stdin_exports = {};
module.exports.bla = { a: 1 };
module.exports = __toCommonJS(stdin_exports);

I tried to spot the change in the diff, but I only found this: v0.14.3...v0.14.5#diff-7c136e0e2ecad99566cc25cf7268b0537dadfe387e46eea3b7d82886e8875d84R1539, which is checking for an import, not an export.

Thanks for your work 🙏, do you think this is a bug or is this another interop ambiguity?

[evanw]

Mixed CommonJS and ESM exports in the same file is not something esbuild is designed for. I added a warning in #1887 but that only checks for people doing module.exports = { a: 1 } not module.exports.bla = { a: 1 }. I can extend the warning to cover this case too.

[p0wl]

Awesome, thanks! We'll upgrade to only use ESM syntax, which wasn't possible before we used esbuild in our project 🎉