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The benefits of Typescript in a team environment for building and maintaining production-worthy codebases are nothing new. At Cleo we committed early to developing our web and mobile apps with Typescript to give us the safety of static typing when developing our product.
One issue we faced building SPAs with Typescript was additional boilerplate when combined with other paradigms, specifically state management and Redux. The Redux docs[1] give a good starting point for Typescript integration, but their approach remains convoluted. We wanted a way of simplifying this integration to minimise the effort of achieving full type coverage within our codebase.
We'll create a simple app for displaying a customer's transactions to show how we can create type safety alongside Redux.
We define a new type for our transaction, and the store's state.
// types.ts
type Transaction = {
 id: string;
 description: string;
 amount: number;
 categoryId: Category['id'];
};
type State = {
transactions: Transaction[];
};
We use the typesafe-actions library to generate typed actions. Defining a new params type for when we dispatch the action.
// actions.ts
import { createStandardAction } from 'typesafe-actions';
import { Transaction } from "../types";
type SetTransactionDescriptionParams = { transactionId: Transaction['id'], description: Transaction['description'] };
export const setTransactionDescription = createStandardAction('SET_TRANSACTION_DESCRIPTION')<SetTransactionDescriptionParams>();
export const deleteTransaction = createStandardAction('DELETE_TRANSACTION')<Transaction['id']>();
The following shows the type safety we get when (incorrectly) dispatching actions:
Where typesafe-action starts to shine.
The dispatched action is typed with the ActionType helper. This builds a union type of the actions:
action: ActionType<typeof transactionListActions>
// evaluates to
{ type: 'SET_TRANSACTION_DESCRIPTION', payload: SetTransactionDescriptionParams } || { type: 'DELETE_TRANSACTION', payload: Transaction['id'] }
We use a switch statement on the action's type, and the getType helper to extract the action's type. This allows the payload's type to be correctly inferred & used within the scope.
// reducer.ts
import * as transactionListActions from './actions';
import { ActionType, getType } from 'typesafe-actions';
import { Category, Transaction } from "../types";
export const transactionListReducer = (state: TransactionListState, action: ActionType<typeof transactionListActions>): TransactionListState => {
 switch (action.type) {
  case getType(transactionListActions.setTransactionCategory): //getType evaluates to 'SET_TRANSACTION_DESCRIPTION'
// Payload is now typed correctly
   const { transactionId, categoryId } = action.payload;
...
  case getType(transactionListActions.deleteTransaction):
// Payload is now typed correctly
   const transactionId = action.payload;
...
  default:
   return state;
 }
};
The following shows the payload type suggestions we get when within a case.
Simply type the state
// selectors.ts
import { TransactionListState } from "./reducer";
import { Transaction } from "../types";
export const selectTransactions = (state: TransactionListState) => state.transactions;
export const selectCategories = (state: TransactionListState) => state.categories;
export const selectTransaction = (state: TransactionListState, transactionId: Transaction['id'] ) =>
state.transactions.find(transaction => transaction.id === transactionId);
You’re feeling nervous about your money right now, but you probably don’t need to worry.
This blog post is adapted from a talk I recently gave to alumni from Makers Academy on career progression as a Software Engineer.
Be right back. Going for a stupid walk for my stupid mental health.Â