Chapter 01) Node.jsプラットフォームについて

概要

1.2 Node.js 6とES 2015

1.2.1 let,constキーワード

// var 사용 예
if (false) {
  var x = "hello";
}
console.log(x); // undefined
//------------------------------------
// let 사용 예
if (false) {
  let x = "hello";
}
console.log(x); // ReferenceError: x is not defined

// Error
const x = 'This will never change';
x = '...'; // TypeError: Assignment to constant variable
//------------------------------------
// Not Error
const x = {}
x.name = 'John';

1.2.2矢印関数

const numbers = [2, 6, 7, 8, 1];
// 생략 가능 예
const even = numbers.filter(x => x%2 === 0);
// 생략 불가능 예
const even = numbers.filter(x => {
  if (x%2 === 0) {
    console.log(x + ' is even!');
    return true;
  }
});

1.2.3類文法

class Person {
  constructor (name, surname, age) {
    this.name = name;
    this.surname = surname;
    this.age = age;
  }
  
  getFullname () {
    return this.name + ' ' + this.surname;
  }
  
  static older (person1, person2) {
    return (person1.age >= person2.age) ? person1 : person2;
  }
}

class PersonWithMiddlename extends Person {
  constructor (name, middlename, surname, age) {
    super(name, surname, age);
    this.middlename = middlename;
  }
  
  getFullName() {
    return this.name + '' + this.middlename + '' + this.surname;
  }
}

1.2.4強化されたオブジェクトシリーズ

const namespace = '-webkit-';
const style = {
  [namespace + 'box-sizing'] : 'border-box',
  [namespace + 'box-shadow'] : '10px10px5px #888888'
};

const person = {
  name: 'George',
  surname: 'Boole',
  
  get fullname () {
    return this.name + '' + this.surname;
  }
  
  set fullname (fullname) {
    let parts = fullname.split('');
    this.name = parts[0];
    this.surname = parts[1];
  }
};

console.log(person.fullname); // George Boole
console.log(person.fullname = 'Alan Turing');
console.log(person.name); // Alan

// Map
const profiles = new Map();
profiles.set('twitter', '@adalovelace');
profiles.set('facebook', 'adalovelace');
profiles.set('googleplus', 'ada');

profiles.size; // 3
profiles.has('twitter'); // true
profiles.get('twitter'); // "@adalovelace"
profiles.has('youtube'); // false
profiles.delete('facebook');
profiles.has('facebook'); // false
profiles.get('facebook'); // undefined
for (const entry of profiles) {
  console.log(entry);
}

// Set
const s = new Set([0, 1, 2, 3]);
s.add(3);
s.size; // 4
s.delete(0);
s.has(0); //false

for (const entry of s) {
  console.log(entry);
}

1.2.6 WeakMapとWeakSet Collection

// WeakMap
let obj = {};
const map = new WeakMap();
map.set(obj, {key: "some_value"});
console.log(map.get(obj)); // {key: "som_value"}
obj = undefined; // 다음 가비지 컬렉트 사이클에서 맴에 관련된 객체와 데이터가 정리됨

// WeakSet
let obj1 = {key: "val1"};
let obj2 = {key: "val2"};
const set = new WeakSet([obj1, obj2]);
console.log(set.has(obj1)); // true
obj1 = undefined; // obj1이 set에서 제거됨
console.log(set.has(obj1)); // false

1.2.7テンプレート表記法

const name = "Leonardo";
const interests = ["arts", "architecture", "science", "music", "mathematics"];
const birth = { year: 1452, place: "Florence" };
const text = `${name} was an Italian polymath interested in many topics such as ${interests.join(', ')}. He was born in ${birth.year} in ${birth.place}`
console.log(text);

1.2.8 ES 2015のその他の機能