jasonleehodges

export interface Noun {
  id: string;
  created?: number;
  name: string;
}

export interface Character extends Noun {
  gender: Gender;
}

jasonleehodges

const myCharacter: Character = api.get<Character>(`/character/${characterId}`);

if(myCharacter.metadata.type === 'Hero') {
  // this will blow up if there is no "metadata" property on the "myCharacter" object
  // Cannot read property 'type' of undefined
}

if(myCharacter?.metadata?.type === 'Hero') {
  // the optional chaining operator will default the entire expression to undefined
  // if any property along the way is undefined. This condition will evaluate to false

jasonleehodges

declare module 'exampleModule';

jasonleehodges

Could not find a declaration file for module 'exampleModule'. '.../node_modules/exampleModule.js' implicitly has an 'any' type.
  Try `npm i --save-dev @types/exampleModule` if it exists or add a new declaration (.d.ts) file containing `declare module 'exampleModule';`

jasonleehodges

function add(a: number, b: number): number {
  /*
    Let's assume there is a bunch of logic here which makes
    the error message in this function less obvious
  */
  return 'a' + 'b';
}

jasonleehodges

No overload matches this call.
  Overload 1 of 2, '(actionCreator: ActionCreatorWithPayload<Partial<Character>, string>, reducer: CaseReducer<State, { payload: Partial<Character>; type: string; }>): ActionReducerMapBuilder<...>', gave the following error.
    Type '(WritableDraft<Partial<Character>> | "123")[]' is not assignable to type 'WritableDraft<Partial<Character>>[]'.
      Type 'WritableDraft<Partial<Character>> | "123"' is not assignable to type 'WritableDraft<Partial<Character>>'.
        Type '"123"' has no properties in common with type 'WritableDraft<Partial<Character>>'.
  Overload 2 of 2, '(type: string, reducer: CaseReducer<State, { payload: Partial<Character>; type: string; }>): ActionReducerMapBuilder<State>', gave the following error.
    Argument of type 'ActionCreatorWithPayload<Partial<Character>, string>' is not assignable to parameter of type 'string'.

jasonleehodges

import React from "react";
import { useSelector } from "react-redux";
import {selectCountFormatted} from './selectors'

export const Counter = () => {
  const countFormattedAsDollars = useSelector(selectCountFormatted('$'));
  
...

jasonleehodges

export const selectCount = (state: RootState) => state.counter.value;
export const selectCountFormatted =
  (formatter: "$" | "%") => (state: RootState) =>
    `${formatter === "$" ? "$" : ""}${selectCount(state)}${
      formatter === "%" ? "%" : ""
    }`;
export const selectCountAndStatus = (state: RootState) =>
  `The current count is ${selectCount(state)} and the status is ${
    state.counter.status
  }`;

jasonleehodges

export const Counter = () => {
  const count = useSelector((state: RootState) => state.counter.value)

  return (
    <div>
      
   ...

jasonleehodges

import {
  CounterState,
  decrement,
  increment,
  incrementByAmount,
  testableCounterReducer,
} from "./testableCounterReducer";

describe("testable counter reducer", () => {
  const initialState: CounterState = {