TypeScript

Generic Type

Generics let you parameterize types so they can be reused across different shapes.

Common generic utilities and examples:

• Array<ItemType>
• Promise<ReturnedType>
• Readonly<T>
• Partial<T>
• Record<KeyType, ValueType>

Example:

function wrap<T>(value: T): { data: T } {
  return { data: value };
}

const r1 = wrap<string>('hello');
const r2 = wrap(123); // T is inferred as number

What are the primitive types in TypeScript?

• string, number, boolean, bigint, symbol, null, undefined.

What is any type, and when to use it?

• any disables type checking for a value. Use sparingly.
• Implicit any can occur when the compiler cannot infer a type and no annotation is provided.
• Prefer unknown or specific types to keep safety.

What is void, and when to use the void type?

• void indicates a function returns no value.

function log(msg: string): void {
  console.log(msg);
}

What is an unknown type, and when to use it in TypeScript?

• unknown is a type-safe counterpart of any.
• You can assign anything to unknown, but you must narrow it before using it.

function handle(input: unknown) {
  if (typeof input === 'string') {
    // narrowed to string
    console.log(input.toUpperCase());
  }
}

Note to Create Strongly typed Component Events and Event Handlers

• Inline event handlers often infer correct types automatically.
• Named handlers may require explicit annotations.
• Tip: hover handlers in your editor to see inferred types.

function Input() {
  const onKeyDown = (e: React.KeyboardEvent<HTMLInputElement>) => {
    console.log(e.key);
  };

  return <input type="text" onKeyDown={onKeyDown} />;
}

"<input type=""text"" onKeyDown={e:React.KeyboardEvent<HTMLInputElement> => console.log(e.key)} /> What is wrong?"

You must wrap the parameter and annotation in parentheses.

Correct:

<input
  type="text"
  onKeyDown={(e: React.KeyboardEvent<HTMLInputElement>) => console.log(e.key)}
/>

declare type for actions and dispatchs of redux-thunk

import { AnyAction } from 'redux';
import { ThunkAction, ThunkDispatch } from 'redux-thunk';

type AppState = { /* ... */ };

export type AppThunk<ReturnType = void> = ThunkAction<
  ReturnType,
  AppState,
  unknown,
  AnyAction
>;

export type AppDispatch = ThunkDispatch<AppState, unknown, AnyAction>;

declare enum type

enum Status {
  Idle = 'idle',
  Loading = 'loading',
  Success = 'success',
  Error = 'error',
}

function setStatus(s: Status) {}

setStatus(Status.Loading);