migrate reducers/themes to TypeScript (#23716)

Co-authored-by: tae <taeshindefv@gmailcom>

packages/gatsby/src/redux/reducers/index.js

@@ -4,6 +4,7 @@ import { redirectsReducer } from "./redirects"
 import { staticQueryComponentsReducer } from "./static-query-components"
 import { statusReducer } from "./status"
 import { webpackReducer } from "./webpack"
+import { themesReducer } from "./themes"
 import { webpackCompilationHashReducer } from "./webpack-compilation-hash"
 import { reducer as logReducer } from "gatsby-cli/lib/reporter/redux/reducer"
 
@@ -69,7 +70,7 @@ module.exports = {
   redirects: redirectsReducer,
   babelrc: require(`./babelrc`),
   schemaCustomization: require(`./schema-customization`),
-  themes: require(`./themes`),
+  themes: themesReducer,
   logs: logReducer,
   inferenceMetadata: require(`./inference-metadata`),
   pageDataStats: require(`./page-data-stats`),

packages/gatsby/src/redux/reducers/themes.ts

@@ -0,0 +1,17 @@
+import { IGatsbyState, ISetResolvedThemesAction } from "../types"
+
+export const themesReducer = (
+  state: IGatsbyState["themes"] = {},
+  action: ISetResolvedThemesAction
+): IGatsbyState["themes"] => {
+  switch (action.type) {
+    case `SET_RESOLVED_THEMES`:
+      return {
+        ...state,
+        themes: action.payload,
+      }
+
+    default:
+      return state
+  }
+}

packages/gatsby/src/redux/types.ts

@@ -380,6 +380,11 @@ export interface ICreateRedirectAction {
   payload: IRedirect
 }
 
+export interface ISetResolvedThemesAction {
+  type: `SET_RESOLVED_THEMES`
+  payload: IRedirect
+}
+
 export interface IDeleteCacheAction {
   type: `DELETE_CACHE`
 }