tswistak

{
  "compilerOptions": {
    "target": "es5",
    "module": "commonjs",
    "lib": ["es2015", "dom"],
    "outDir": "../../lib/client",
    "composite": true,
    "strict": true,
    "esModuleInterop": true
  },

tswistak

type Props = {
  title?: string;
};

class Heading extends React.Component<Props> {
  static defaultProps = {
    title: 'Hello!'
  };
  render() {
    const title = this.props.title;

tswistak

type Props = {
  title?: string;
};

class Heading extends React.Component<Props> {
  static defaultProps = {
    title: 'Hello!'
  };
  render() {
    const title = this.props.title;

tswistak

import * as React from 'react';
import * as ReactDOM from 'react-dom';

class Heading extends React.Component {
  render() {
    return <h1>{this.props.title.toUpperCase()}</h1>
  }
}

Heading.defaultProps = { title: 'Hello!' };

tswistak

const a1: any = 'a';
const a2: unknown = 'a';
a1.length; // = 1
a2.length; // compiler error
a1(); // error during execution
a2(); // compiler error
new a1(); // error during execution
new a2(); // compiler error
const a3 = a1 + 3; // = 'a3'
const a4 = a2 + 3; // compiler error

tswistak

function compose2<T1 extends any[], T1R, T2>(f1: (...args1: T1) => T1R,  f2: (arg: T1R) => T2) {
  return (...a: T1) => f2(f1(...a));
}

const add = (x: number, y: number) => x + y;
const sqr = (x: number) => x * x;
const addAndSqr = compose2(add, sqr);

addAndSqr(1, 2); // valid
addAndSqr('a', 2); // invalid type

tswistak

function hello(...args: [string, number, number, ...string[]]): string {
    return args.join(',');
}

tswistak

// JavaScript
function jsHello() {
  return [...arguments].join(',');
}

// JavaScript, second way
function jsHello1(...args) {
  return args.join(',');
}

tswistak

type Strings = [...string];
type Empty = [];
const c1: Strings = ['a', 'b'];
const c2: Strings = [];
const c3: Empty = [];
const c4: Empty = ['a', 'b']; // invalid type

tswistak

type StringAndNumbers = [string, ...number[]];
const b1: StringAndNumbers = ['a', 1, 2, 3];
const b2: StringAndNumbers = ['a'];