Add comments to the thunk middleware implementation

src/index.ts

@@ -9,28 +9,40 @@ export type {
   ThunkMiddleware
 } from './types'
 
+// A function that accepts a potential "extra argument" value to be injected later,
+// and returns an instance of the thunk middleware that uses that value
 function createThunkMiddleware<
   TState = {},
   TBasicAction extends Action = AnyAction,
   TExtraThunkArg = undefined
 >(extraArgument?: TExtraThunkArg) {
+  // Standard Redux middleware definition pattern:
+  // See: https://redux.js.org/tutorials/fundamentals/part-4-store#writing-custom-middleware
   const middleware: ThunkMiddleware<TState, TBasicAction, TExtraThunkArg> =
     ({ dispatch, getState }) =>
     next =>
     action => {
+      // The thunk middleware looks for any functions that were passed to `store.dispatch`.
+      // If this "action" is really a function, call it and return the result.
       if (typeof action === 'function') {
+        // Inject the store's `dispatch` and `getState` methods, as well as any "extra arg"
         return action(dispatch, getState, extraArgument)
       }
 
+      // Otherwise, pass the action down the middleware chain as usual
       return next(action)
     }
   return middleware
 }
 
+// The standard thunk middleware has no extra argument included
 const thunk = createThunkMiddleware()
+// Attach the factory function so users can create a customized version
+// with whatever "extra arg" they want to inject into their thunks
 // @ts-ignore
 thunk.withExtraArgument = createThunkMiddleware
 
+// Convince TS that the default export has the right type for `withExtraArgument`
 export default thunk as typeof thunk &
   ThunkMiddleware & {
     withExtraArgument<

src/types.ts

@@ -37,7 +37,7 @@ export interface ThunkDispatch<
  *
  * @template TReturnType The return type of the thunk's inner function
  * @template TState The redux state
- * @template TExtraThunkARg Optional extra argument passed to the inner function
+ * @template TExtraThunkArg Optional extra argument passed to the inner function
  * (if specified when setting up the Thunk middleware)
  * @template TBasicAction The (non-thunk) actions that can be dispatched.
  */