flutter create -i swift -a kotlin my_flutter_app

const fallbackImage = '...';

type Props = {
  src: JSX.IntrinsicElements['img']['src'];
  alt?: JSX.IntrinsicElements['img']['alt'];
  srcOnError?: JSX.IntrinsicElements['img']['src'];
  className?: JSX.IntrinsicElements['img']['className'];
};

function Component(props: Props) {
  const { src, alt, srcOnError, className } = props;
  const srcFileName = src?.split('/').pop()?.split('.').shift();
  return (
    <img
      className={className}
      src={src}
      alt={alt || srcFileName}
      onError={(e) => {
        e.currentTarget.onerror = null;
        e.currentTarget.src = srcOnError ?? fallbackImage;
      }}
    />
  );
}

export const Image = Component;

open -a Simulator

または

open /Applications/Xcode.app/Contents/Developer/Applications/Simulator.app

const items = ['hello', null, 'world'];

const filtered = items.filter((item) => item !== null);
// filtered: (string | null)[]

class Calculator {
  #number;

  constructor(initial = 0) {
    this.#number = initial;
  }

  print() {
    console.log(this.#number);
  }

  add(num) {
    this.#number += num;
    return this;
  }

  subtract(num) {
    this.#number -= num;
    return this;
  }

  multiply(num) {
    this.#number *= num;
    return this;
  }

  divide(num) {
    this.#number /= num;
    return this;
  }
}

new Calculator(9).add(5).subtract(4).multiply(3).divide(2).print(); // 15

function foo<T>(arg: T) {
  return arg;
}

function bar<T extends object>(arg: T) {
  return arg;
}

type FooReturn = ReturnType<typeof foo>; // unknown

type BarReturn = ReturnType<typeof bar>; // object

type Person = {
  name: string;
  age: number;
  isAdult: boolean;
  siblings: string[];
  birthplace?: string;
  height?: number;
  hasPartner?: boolean;
  friends?: string[];
};

type OptionalPropertyKeys<T> = {
  [K in keyof T]-?: undefined extends T[K] ? K : never;
}[keyof T];

type OptionalPersonPropertyKeys = OptionalPropertyKeys<Person>;
// 👉 "birthplace" | "height" | "hasPartner" | "friends"

type OptionalPersonProperty = Pick<Person, OptionalPersonPropertyKeys>;
// 👉 {
//      birthplace?: string;
//      height?: number;
//      hasPartner?: boolean;
//      friends?: string[];
//    }

/**
 * ということでこうして使いましょう。
 */
type OptionalProperty<T> = Pick<
  T,
  {
    [K in keyof T]: undefined extends T[K] ? K : never;
  }[keyof T]
>;

import { useState, useEffect } from 'react';

export default function Component() {
  const [int, setInt] = useState<number>();

  useEffect(() => setInt(1), []);
  useEffect(() => setInt(3), []);
  useEffect(() => setInt(2), []);

  console.log(int);

  return <></>;
}

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2

import { useRef, useEffect } from 'react';

function Component() {
  const ref = useRef();

  useEffect(() => {
    ref.current.innerHTML = 'Hello, world.';
  }, []);

  return <div ref={ref} />;
}

import { useCallback } from 'react';

function Component() {
  const setRef = useCallback((node) => {
    node.innerHTML = 'Hello, world.';
  }, []);

  return <div ref={setRef} />;
}

import { useRef, useEffect } from 'react';

function Component() {
  const ref = useRef();

  useEffect(() => {
    ref.current.innerHTML = 'Hello, world.';
  }, []);

  const handleOnClick = () => {
    ref.current.innerHTML = 'Goodbye, world.';
  };

  return <div ref={ref} onClick={handleOnClick} />;
}

import { useRef, useCallback } from 'react';

function Component() {
  const ref = useRef();

  const setRef = useCallback((node) => {
    node.innerHTML = 'Hello, world.';
    ref.current = node;
  }, []);

  const handleOnClick = () => {
    ref.current.innerHTML = 'Goodbye, world.';
  };

  return <div ref={setRef} onClick={handleOnClick} />;
}

function Component() {
  return <div ref={(ref) => console.log(ref)} />; // <div></div>
}

import { ChildB } from './ChildB';
import { ChildA } from './ChildA';

console.log('Parent | Outer Top');

export function Parent() {
  console.log('Parent | Inner');
  return (
    <>
      <ChildA />
      <ChildB />
    </>
  );
}

console.log('Parent | Outer Bottom');

type Greet = 'hello' | 'goodbye';

const greet: Greet = 'hello';

import { initializeApp } from 'firebase/app';
import { getAuth } from 'firebase/auth';
import { getFirestore } from 'firebase/firestore';
import { getFunctions } from 'firebase/functions';

/** 引数を Date オブジェクトへ変換する。Date として解釈できなかった場合は引数をそのまま返す。 */
const tryParseToDate = (arg) => {
  if (arg instanceof Date) {
    return arg;
  }
  if (typeof arg !== 'string' && typeof arg !== 'number') {
    return arg;
  }
  const maybeDate = new Date(arg);
  const invalidDate = Number.isNaN(maybeDate.getTime());
  return invalidDate ? arg : maybeDate;
};