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[[TypeScript and React]]


  • ignore a rule for the whole file
/* eslint-disable rule-name */
// @ts-nocheck (ignores the whole file)
  • ignore the rule for one line
/* eslint-disable-next-line rule-name */

// @ts-ignore (ignores one line)

Checking types

instanceof vs typeof

JS built-ins

  • instanceof for custom types
  • typeof for the simple built-in types

User Defined Type Guards

Return type isn't a boolean!

function isFish(pet: Fish | Bird): pet is Fish {
    return (pet as Fish).swim !== undefined;

User defined type guards aren't type checked!

The complete guide to safe type narrowing in TypeScript

Design choice: Suggestion: check narrowed type in user-defined type guards · Issue #29980 · microsoft/TypeScript · GitHub

const isNumber = (num: unknown): num is number => {
  return typeof num === 'string';

const nums = [1, '2', 3]
const onlyNumbers: number[] = nums.filter(isNumber)
console.log(onlyNumbers) // ["2"]

Downcast function: Alternative 1 to user defined type guard

function toNumber(arg: unknown, fallback: number): number {  
    if (typeof arg === 'number') return arg;
    if (typeof arg === 'string') return Number(arg);
    return fallback;

Discriminating union: alternative 2 to user defined type guard

User-Defined Type Guards aren't safe

Narrow down from a union

type Hotdog = {
  _tag: 'Hotdog';

type Burger = {
  _tag: 'Burger';

type Food = Hotdog | Burger;

// This little helper provides a safer way to narrow the type.
const is = <A, B extends A>(fn: (a: A) => B | undefined) => {
  return (a: A): a is B => typeof fn(a) !== 'undefined';

const isBurger = is<Food, Burger>(food => (food._tag === 'Burger' ? food : undefined));

const foods = [{ _tag: 'Hotdog' }, { _tag: 'Burger' }];

const a = foods.filter(isBurger);

Type Generation


Convert something to a string



  • Inferred generic: (from the params)
  • example: React.useState()
    • explicitly: React.useState<UnknownObject>({})

Importing a type that's the same name as the class

When working with progressbar.js, I was getting errors

When I tried importing the Shape,

import ProgressBar, { PathDrawingOptions, Shape } from "progressbar.js";

it was still importing the Shape object and not the type

VSCode complains because it knows that Shape isn't a type - it suggests typeof Shape which isn't right because ????


There's a separate import for the Shape

import type Shape from "progressbar.js/shape";

declare keyword

Example: importing from a JS library

Declare that some variable that you has this type

declare let module: any;

declare const module = { doSomething: () => boolean };

Last update: 2023-04-24