Unable to preserve dynamic imports with transform or transformSync API

[marvinhagemeister]

I'm currently using to esbuild to hook into node's module loader via esbuild-register to compile files on demand when they are loaded in node. Internally, it relies on transformSync to transpile files sent by node's internal module loader.

The app is mainly written in commonjs, but loads some esm files via dynamic import statements. To allow node to load esm files from commonjs one must use the dynamic import statement. In esbuild this is supported via the --platform=node flag in cli or via a build property of the same name for build and buildSync APIs.

But no such option exists for transform or transformSync. Those APIs only support setting format=cjs which transpiles every import, including dynamic import statements. That's great for browsers but unwanted in node environments. So there needs to be some way to preserve dynamic import statements via transform and transformSync.

Input code:

// input
export const foo = () => import('./bar.mjs')

Expected output code:

const foo = () => import("./bar.mjs")));

Actual output code:

// ...snip bunch of esm interop code
const foo = () => Promise.resolve().then(() => __toModule(require("./bar.mjs")));

[evanw]

I think at some point when I was writing this I didn't realize that import() was a regular function in script mode. Also the TypeScript compiler converts import() to require() when the output format is CommonJS, so I have done that as well. I agree that this would be something good to change. However, this seems like a breaking change to me, so it should wait until the next breaking change release.

[marvinhagemeister]

Agree, I think that's an error in TypeScript too. There is a relevant issue on their tracker: microsoft/TypeScript#43329