ES6小技巧整理

const required = () => {throw new Error('Missing parameter')};

const add = (a = required(), b = required()) => a + b;

add(1, 2) //3
add(1) // Error: Missing parameter.

const numbers = [10, 20, 30, 40];
const doubledOver50 = numbers.reduce((finalList, num) => { 
 num = num * 2;  
 if (num > 50) {
  finalList.push(num);
 } 
 return finalList;
}, []);
doubledOver50; // [60, 80]

//Returns 0 if balanced.
const isParensBalanced = (str) => {
 return str.split('').reduce((counter, char) => {
  if(counter < 0) { //matched ")" before "("
   return counter;
  } else if(char === '(') {
   return ++counter;
  } else if(char === ')') { 
   return --counter;
  } else { //matched some other char
   return counter;
  }
  
 }, 0); //<-- starting value of the counter}
isParensBalanced('(())') // 0 <-- balanced
isParensBalanced('(asdfds)') //0 <-- balanced
isParensBalanced('(()') // 1 <-- not balanced
isParensBalanced(')(') // -1 <-- not balanced

var cars = ['BMW','Benz', 'Benz', 'Tesla', 'BMW', 'Toyota'];
var carsObj = cars.reduce(function (obj, name) { 
  obj[name] = obj[name] ? ++obj[name] : 1;
 return obj;
}, {});
carsObj; // => { BMW: 2, Benz: 2, Tesla: 1, Toyota: 1 }

let {_internal, tooBig, ...cleanObject} = {el1: '1', _internal:"secret", tooBig:{}, el2: '2', el3: '3'};

console.log(cleanObject); // {el1: '1', el2: '2', el3: '3'}

var car = {
 model: 'bmw 2018',
 engine: {
  v6: true,
  turbo: true,
  vin: 12345
 }
}
const modelAndVIN = ({model, engine: {vin}}) => {
 console.log(`model: ${model} vin: ${vin}`);
}
modelAndVIN(car); // => model: bmw 2018 vin: 12345

let object1 = { a:1, b:2,c:3 }
let object2 = { b:30, c:40, d:50}
let merged = {…object1, …object2} //spread and re-add into merged
console.log(merged) // {a:1, b:30, c:40, d:50}

let arr = [1, 1, 2, 2, 3, 3];
let deduped = [...new Set(arr)] // [1, 2, 3]

let mySet = new Set([1,2, 3, 4, 5]);
var filtered = [...mySet].filter((x) => x > 3) // [4, 5]

let param1 = 1;
let param2 = 2;
//swap and assign param1 & param2 each others values
[param1, param2] = [param2, param1];
console.log(param1) // 2
console.log(param2) // 1

async function getFullPost(){
 return await Promise.all([
   fetch('/post'),
   fetch('/comments')
 ]);
}
const [post, comments] = getFullPost();