# turbopack-ecmascript-runtime This crate contains Turbopack's ECMAScript runtimes. These include: - Development runtimes for the browser, Node.js, and Edge-like runtimes; - Production runtimes for the browser, Node.js, and Edge-like runtimes (only Node.js is implemented for now). ## `` The TypeScript files corresponding to the runtime itself all use `` instead of `import`/`export` to import dependencies. This is because the runtime doesn't use a module system. Instead, the files are concatenated together in a specific order. As such, the `` statements more closely map to the way the runtime is actually built. They also allow us to refer to top-level declarations in another file without having to `import`/`export` them, which makes no sense in the context of a runtime. ## Why is everything in one crate? The runtime-agnostic code (`js/src/shared`) was originally placed in `turbopack-ecmascript`, and the runtime-specific code (`js/src/{build,dev}`) in `turbopack-{build,dev}`. However, `` statements only support relative paths. You can't refer to a file in a dependency. For the typings to work properly, and for them to be usable from outside of this repo (e.g. in the Next.js repo), it's much easier to have everything in one package. ## Why so many `tsconfig.json`? Since different runtimes are meant to run in different environments, they use different `tsconfig.json` files to customize what APIs are available to them. For example, the browser runtime can use `window` and `document`, but the Node.js runtime can't. All of these `tsconfig.json` files extend `tsconfig.base.json`, which contains the common configuration for all runtimes.