// www.justjavascript.com
typeof null === 'object'; // true

const arr = [1, 2, 3];
return [0, ...arr]; // [0, 1, 2, 3]

const arr = [1, 2, 3];
arr.unshift(0); // returns new length: 4
return arr; // [0, 1, 2, 3]

const arr = [1, 2, 3];
return [...arr, 4]; // [1, 2, 3, 4]

const arr = [1, 2, 3];
arr.push(4); // returns new length: 4
return arr; // [1, 2, 3, 4]

// https://stackoverflow.com/a/1026087/1013
function capitalizeFirstLetter(string) {
	return string.charAt(0).toUpperCase() + string.slice(1);
}

// https://stackoverflow.com/a/5915122/1013
function rand(items) {
	return items[Math.floor(Math.random() * items.length)];
}

// https://developer.mozilla.org/en-US/docs/Glossary/Falsy
function f() {
	// prettier-ignore
	return [
      Boolean(false),
      Boolean(''),
      Boolean(``),
      Boolean(""),
      Boolean(0),
      Boolean(0n),
      Boolean(-0),
      Boolean(-0n),
      Boolean(NaN),
      Boolean(null),
      Boolean(undefined)]
}
f(); // returns array of `false`s

// https://developer.mozilla.org/en-US/docs/Glossary/Falsy
function f() {
	return [Boolean(new Boolean(false)), Boolean('false'), Boolean({}), Boolean([]), Boolean(-1)];
}
f(); // [true, true, true, true, true]

// It's basically !(!0) that is converted to !(true) which in turn becomes false
!!0; // false

// https://stackoverflow.com/questions/1758576/multiple-left-hand-assignment-with-javascript/1758912#1758912
function f() {
	//prettier-ignore
	var a1 = a2 = 1 // this is like var a1 = (a2 = 1)
	//prettier-ignore
	var b1 = 1, b2 = 1
	return [window.a1, window.a2, window.b1, window.b2];
}

f(); // [undefined, 1, undefined, undefined]

// 2020-5-8
// bigint at 70% https://caniuse.com/#feat=bigint
// symbol 93% https://caniuse.com/#feat=mdn-javascript_builtins_symbol

typeof undefined; // "undefined"
typeof null; // "object"
typeof true; // "boolean"
typeof 1; // "number"
typeof 1.1; // "number"
typeof 2n; // "bigint"
typeof 'some text'; // "string
typeof Symbol('some value'); // "symbol"
typeof function g() {}; // "function"
typeof {}; // "object"
typeof []; // "object"
typeof new Date(); // "object"


typeof Boolean // "function"

typeof typeof 10; // "string" (typeof returns string "number")

// What the heck is the event loop https://www.youtube.com/watch?v=8aGhZQkoFbQ
// Jaffa Cake Asia: https://www.youtube.com/watch?v=cCOL7MC4Pl0
// Sync BEFORE promises (microtask) BEFORE settimeout (macrotask)
const result = [];
function f() {
	setTimeout(() => {
		result.push('settimeout');
	}, 0);
	Promise.resolve().then(() => result.push('promise'));
	Promise.resolve().then(result.push('this is not a function, so it executes sync'));
	result.push('sync');
}
f();
setTimeout(() => {
	console.log(result);
}, 1);

// https://javascript.info/bigint

2n > 1; // true
1n > 1; // false
1n == 1; // true
1n === 1; // false
1n < 1; // false

//  ASI https://stackoverflow.com/a/2846298/1013
function f() {
	return {
		a: 'hello',
	};
}

function g() {
	return;
	{
		a: 'hello';
	}
}

f(); // {a: "hello"}
g(); // undefined

//
function f() {
	return 2 / 0 === Infinity;
}
f(); // true

Number.isNaN(0 / 0); // true
Number.isNaN('hey' / 0); // true
Number.isNaN('hey'); // false

// https://0.30000000000000004.com/
// For some reason JS is famous for this, but there are many languages
// that do the same.
// A way to think of it is it's like 1/3 is 0.3333(3) on base10.
// In base2 this thing happens for 0.1 + 0.2.

0.1 + 0.2 == 0.3; // false
0.1 + 0.2 === 0.3; // false
0.1 + 0.2; // 0.30000000000000004

// The same word if written back and forth.
// https://www.toptal.com/javascript/interview-questions
function f(str) {
	return str === str.split('').reverse().join('');
}

f('aba'); // true
f('aab'); // false

useEffect(() => {
	const timerID = setTimeout(() => {
		// do stuff
	}, 2000);
	return clearTimeout(timerID);
}, []);