# @smithy/core

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### :warning: Internal API :warning:

> This is an internal package.
> That means this is used as a dependency for other, public packages, but
> should not be taken directly as a dependency in your application's `package.json`.

> If you are updating the version of this package, for example to bring in a
> bug-fix, you should do so by updating your application lockfile with
> e.g. `npm up @scope/package` or equivalent command in another
> package manager, rather than taking a direct dependency.

---

This package provides common or core functionality for generic Smithy clients.

You do not need to explicitly install this package, since it will be installed during code generation if used.

## Development of `@smithy/core` submodules

Core submodules are organized for distribution via the `package.json` `exports` field.

`exports` is supported by default by the latest Node.js, webpack, and esbuild. For react-native, it can be
enabled via instructions found at [reactnative.dev/blog](https://reactnative.dev/blog/2023/06/21/package-exports-support), but we also provide a compatibility redirect.

Think of `@smithy/core` as a mono-package within the monorepo.
It preserves the benefits of modularization, for example to optimize Node.js initialization speed,
while making it easier to have a consistent version of core dependencies, reducing package sprawl when
installing a Smithy runtime client.

### Guide for submodules

- Each `index.ts` file corresponding to the pattern `./src/submodules/<MODULE_NAME>/index.ts` will be
  published as a separate `dist-cjs` bundled submodule index using the `Inliner.js` build script.
- create a folder as `./src/submodules/<SUBMODULE>` including an `index.ts` file and a `README.md` file.
  - The linter will throw an error on missing submodule metadata in `package.json` and the various `tsconfig.json` files, but it will automatically fix them if possible.
- a submodule is equivalent to a standalone `@smithy/<pkg>` package in that importing it in Node.js will resolve a separate bundle.
- submodules may not relatively import files from other submodules. Instead, directly use the `@scope/pkg/submodule` name as the import.
  - The linter will check for this and throw an error.
- To the extent possible, correctly declaring submodule metadata is validated by the linter in `@smithy/core`.
  The linter runs during `yarn build` and also as `yarn lint`.

### When should I create an `@smithy/core/submodule` vs. `@smithy/new-package`?

Keep in mind that the core package is installed by all downstream clients.

If the component functionality is upstream of multiple clients, it is
a good candidate for a core submodule. For example, if `middleware-retry` had been written
after the support for submodules was added, it would have been a submodule.

If the component's functionality is downstream of a client (rare), or only expected to be used by a very small
subset of clients, it could be written as a standalone package.