przemyslawjanpietrzak

type ReadonlyObject<A> = { readonly [K in keyof A]: A[K] };
type DeepReadonlyObject<A> = { readonly [K in keyof A]: DeepReadonly<A[K]> }
  
type X = DeepReadonlyObject<{ key: string, key1: number }>; // { readonly key: any; readonly key1: any; }

przemyslawjanpietrzak

// @ts-check
export const fn = (arg1, arg2, arg3) => 42;

fn(1, 2, 3, 4);  // ERROR Expected 3 arguments, but got 4
document.querySelector(42);  // ERROR Argument of type '42' is not assignable to parameter of type 'string'.

/**
 *
 * @param {string} arg
 * @returns {number}

przemyslawjanpietrzak

const fn = (arg: unknown) => {
  arg.x; // ERROR
  arg > 3; // ERROR
  arg + 1; // ERROR
  arg.toString(); // ERROR
  arg(); // ERROR
  arg === null; // OK
  return arg; // OK
}

przemyslawjanpietrzak

// https://stackoverflow.com/questions/9847580/how-to-detect-safari-chrome-ie-firefox-and-opera-browser
// Opera 8.0+
var isOpera = (!!window.opr && !!opr.addons) || !!window.opera || navigator.userAgent.indexOf(' OPR/') >= 0;

// Firefox 1.0+
var isFirefox = typeof InstallTrigger !== 'undefined';

// Safari 3.0+ "[object HTMLElementConstructor]" 
var isSafari = /constructor/i.test(window.HTMLElement) || (function (p) { return p.toString() === "[object SafariRemoteNotification]"; })(!window['safari'] || (typeof safari !== 'undefined' && safari.pushNotification));

przemyslawjanpietrzak

interface X {
  key1: string;
  key2: { id: number, name: string },
  key3: number;
}

const fn = (arg: X['key2']) => {
    arg.id // number
    arg.name // string
};

przemyslawjanpietrzak

let tuple: [string, number];
tuple = ["hello", 10]; // OK
tuple = [10, "hello"]; // Error
let str = tuple[0]; // string
let num = tuple[1]; // number

przemyslawjanpietrzak

abstract class AbstractClass {
    method() {
        return 42
    }
}

class Class extends AbstractClass {
    method1() {
        return 42;
    }

przemyslawjanpietrzak

const fn = (arg: number, cb: (string) => string) => {
    return 42
}

fn(42, a => a++); // Argument of type '(a: any) => number' is not assignable to parameter of type '(string: any) => string'. Type 'number' is not assignable to type 'string'.

przemyslawjanpietrzak

const fn = (arg: { key: string } & { key1: number }) => 42;

fn({ key: '42' }); // ERROR
fn({ key1: 42 }); // ERROR
fn({ key: '42', key1: 42 }); // GOOD

przemyslawjanpietrzak

// GOOD
const fn = (arg: { [key: string]: number }) => {
    const val =  arg.key1 + arg.key2 + arg.key3; // number
    const val1 = arg.totallyRandomKey; // number
    const val2 = arg['wpłynąłem na suchego przestwór oceanu']; // number
};