docs(decisions-0003): infer types for useLoaderData from loader type using generic

decisions/0003-infer-types-for-useloaderdata-and-useactiondata-from-loader-and-action-via-generics.md

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+# Infer types for `useLoaderData` and `useActionData` from `loader` and `action` via generics
+
+Date: 2022-07-11
+
+Status: accepted
+
+## Context
+
+Goal: End-to-end type safety for `useLoaderData` and `useActionData` with great Developer Experience (DX)
+
+Related discussions:
+- [remix-run/remix#1254](https://github.com/remix-run/remix/pull/1254)
+- [remix-run/remix#3276](https://github.com/remix-run/remix/pull/3276)
+
+---
+
+In Remix v1.6.4, types for both `useLoaderData` and `useActionData` are parameterized with a generic:
+
+```tsx
+type MyLoaderData = {/* ... */}
+type MyActionData = {/* ... */}
+
+export default function Route() {
+  let loaderData = useLoaderData<MyLoaderData>()
+  let actionData = useActionData<MyActionData>()
+  return <div>{/* ... */}</div>
+}
+```
+
+For end-to-end type safety, it is then the user's responsability to make sure that `loader` and `action` also use the same type in the `json` generic:
+
+```ts
+export const loader: LoaderFunction = () => {
+  return json<MyLoaderData>({/* ... */})
+}
+
+export const action: ActionFunction = () => {
+  return json<MyActionData>({/* ... */})
+}
+```
+
+### Diving into `useLoaderData`'s and `useActionData`'s generics
+
+Tracing through the `@remix-run/react` source code (v1.6.4), you'll find that `useLoaderData` returns an `any` type that is implicitly type cast to whatever type gets passed into the `useLoaderData` generic:
+
+```ts
+// https://github.com/remix-run/remix/blob/v1.6.4/packages/remix-react/components.tsx#L1370
+export function useLoaderData<T = AppData>(): T {
+  return useRemixRouteContext().data; // 
+}
+
+// https://github.com/remix-run/remix/blob/v1.6.4/packages/remix-react/components.tsx#L73
+function useRemixRouteContext(): RemixRouteContextType {/* ... */}
+
+// https://github.com/remix-run/remix/blob/v1.6.4/packages/remix-react/components.tsx#L56
+interface RemixRouteContextType {
+  data: AppData;
+  id: string;
+}
+
+// https://github.com/remix-run/remix/blob/v1.6.4/packages/remix-react/data.ts#L4
+export type AppData = any;
+```
+
+Boiling this down, the code looks like:
+
+```ts
+let data: any
+
+// somewhere else, `loader` gets called an sets `data` to some value
+
+function useLoaderData<T>(): T {
+  return data // <-- Typescript casts this `any` to `T`
+}
+```
+
+`useLoaderData` isn't basing its return type on how `data` was set (i.e. the return value of `loader`) nor is it validating the data.
+It's just blindly casting `data` to whatever the user passed in for the generic `T`.
+
+### Issues with current approach
+
+The developer experience is subpar.
+Users are required to write redundant code for the data types that could have been inferred from the arguments to `json`.
+Changes to the data shape require changing _both_ the declared `type` or `interface` as well as the argument to `json`.
+
+Additionally, the current approach encourages users to pass the same type to `json` with the `loader` and to `useLoaderData`, but **this is a footgun**!
+`json` can accept data types like `Date` that are JSON serializable, but `useLoaderData` will return the _serialized_ type:
+
+```ts
+type MyLoaderData = {
+  birthday: Date
+}
+
+export const loader: LoaderFunction = () => {
+  return json<MyLoaderData>({ birthday: new Date('February 15, 1992') })
+}
+
+export default function Route() {
+  let { birthday } = useLoaderData<MyLoaderData>()
+  // ^ `useLoaderData` tricks Typescript into thinking this is a `Date`, when in fact its a `string`!
+}
+```
+
+Again, the same goes for `useActionData`.
+
+### Solution criteria
+
+- Return type of `useLoaderData` and `useActionData` should somehow be inferred from `loader` and `action`, not blindly type cast
+- Return type of `loader` and `action` should be inferred
+  - Necessarily, return type of `json` should be inferred from its input
+
+### Key insight: `loader` and `action` are an _implicit_ inputs
+
+While there's been interest in inferring the types for `useLoaderData` based on `loader`, there was [hesitance to use a Typescript generic to do so](https://github.com/remix-run/remix/pull/3276#issuecomment-1164764821).
+Typescript generics are apt for specifying or inferring types for _inputs_, not for blindly type casting output types.
+
+A key factor in the decision was identifying that `loader` and `action` are _implicit_ inputs of `useLoaderData` and `useActionData`.
+
+In other words, if `loader` and `useLoaderData` were guaranteed to run in the same process (and not cross the network), then we could write `useLoaderData(loader)`, specifying `loader` as an explicit input for `useLoaderData`.
+
+```ts
+// _conceptually_ `loader` is an input for `useLoaderData`
+function useLoaderData<Loader extends LoaderFunction>(loader: Loader) {/*...*/}
+```
+
+Though `loader` and `useLoaderData` exist together in the same file at development-time, `loader` does not exist at runtime in the browser.
+Without the `loader` argument to infer types from, `useLoaderData` needs a way to learn about `loader`'s type at compile-time.
+
+
+Additionally, `loader` and `useLoaderData` are both managed by Remix across the network.
+While its true that Remix doesn't "own" the network in the strictest sense, having `useLoaderData` return data that does not correspond to its `loader` is an exceedingly rare edge-case.
+
+Same goes for `useActionData`.
+
+---
+
+A similar case is how [Prisma](https://www.prisma.io/) infers types from database schemas available at runtime, even though there are (exceedingly rare) edge-cases where that database schema _could_ be mutated after compile-time but before run-time.
+
+## Decision
+
+Explicitly provide type of the implicit `loader` input for `useLoaderData` and then infer the return type for `useLoaderData`.
+Do the same for `action` and `useActionData`.
+
+```ts
+export const loader = async (args: LoaderArgs) => {
+  // ...
+  return json(/*...*/)
+}
+
+export default function Route() {
+  let data = useLoaderData<typeof loader>()
+  // ...
+}
+```
+
+Additionally, the inferred return type for `useLoaderData` will only include serializable (JSON) types.
+
+### Return `unknown` when generic is omitted
+
+Omitting the generic for `useLoaderData` or `useActionData` results in `any` being returned.
+This hides potential type errors from the user.
+Instead, we'll change the return type to `unknown`.
+
+```ts
+type MyLoaderData = {/*...*/}
+
+export default function Route() {
+  let data = useLoaderData()
+  // ^? unknown
+}
+```
+
+Note: Since this would be a breaking change, changing the return type to `unknown` will be slated for v2.
+
+### Deprecate non-inferred types via generics
+
+Passing in a non-inferred type for `useLoaderData` is hiding an unsafe type cast.
+Using the `useLoaderData` in this way will be deprecated in favor of an explicit type cast that clearly communicates the assumptions being made:
+
+```ts
+type MyLoaderData = {/*...*/}
+
+export default function Route() {
+  let dataGeneric = useLoaderData<MyLoaderData>() // <-- will be deprecated
+  let dataCast = useLoaderData() as MyLoaderData // <-- will be deprecated
+}
+```
+
+## Consequences
+
+- Users can continue to provide non-inferred types by type casting the result of `useLoaderData` or `useActionData`
+- Users can opt-in to inferred types by using `typeof loader` or `typeof action` at the generic for `useLoaderData` or `useActionData`.
+- Return types for `loader` and `action` will be the sources-of-truth for the types inferred for `useLoaderData` and `useActionData`.
+- Users do not need to write redundant code to align types across the network
+- Return type of `useLoaderData` and `useActionData` will correspond to the JSON _serialized_ types from `json` calls in `loader` and `action`, eliminating a class of errors.
+- `LoaderFunction` and `ActionFunction` should not be used when opting into type inference as they override the inferred return types.[^1]
+
+
+🚨 Users who opt-in to inferred types **MUST** return a `TypedResponse` from `json` and **MUST NOT** return a bare object:
+
+```ts
+const loader = () => {
+  // NO
+  return { hello: "world" }
+
+  // YES
+  return json({ hello: "world" })
+}
+```
+
+[^1]: The proposed `satisfies` operator for Typescript would let `LoaderFunction` and `ActionFunction` enforce function types while preserving the narrower inferred return type: https://github.com/microsoft/TypeScript/issues/47920