ES6 のシンボルの詳細な紹介 (コード例)

var sym = Symbol();
var sym1 = new Symbol(); // TypeError
sym instanceof Symbol // false

var sym = Symbol('foo');
typeof sym;     // "symbol"

var sym2 = Symbol('foo');
var sym3 = Symbol('foo');

Symbol('foo') === Symbol('foo'); // false

sym | 0 // TypeError
Symbol("foo") + "bar" // TypeError

var obj = {};
obj[Symbol("a")] = "a";
obj[Symbol.for("b")] = "b";
obj["c"] = "c";
obj.d = "d";
for (var i in obj) {
   console.log(i); // logs "c" and "d"
}

JSON.stringify({[Symbol("foo")]: "foo"});  // '{}'

var mySymbol1 = Symbol.for('some key'); 
var mySymbol2 = Symbol.for('some key');
mySymbol1 == mySymbol2 //true

const myPrivateMethod = Symbol("myPrivateMethod");
const myPrivateProperty = Symbol("myPrivateProperty");
const obj = {
    [myPrivateProperty]: "semlinker",
    [myPrivateMethod]() {
        return `Hello ${this[myPrivateProperty]}!!!`;
    },
    hello() {
        console.log(this[myPrivateMethod]()); 
    }
};
console.log(Object.keys(obj));
console.log(obj.hello());

const myPrivateMethod = Symbol("myPrivateMethod");
const myPrivateProperty = Symbol("myPrivateProperty");
class MyClass {
    constructor() {
        this[myPrivateProperty] = "semlinker";
    }
    [myPrivateMethod]() {
        return `Hello ${this[myPrivateProperty]}!!!`;
    }
    hello() {
        console.log(this[myPrivateMethod]()); 
    }
}
const myCls = new MyClass();
console.log(myCls.hello());

class Skill {
    constructor() {
        this.skills = ['Angular', 'React', 'Vue', 'Koa', 'Ionic'];
    }
    [Symbol.iterator]() {
        let index = 0;
        return {
            next: () => {
                const value = this.skills[index++];
                const done = index === this.skills.length + 1;
                return {
                    value,
                    done
                };
            }
        }
    }
}
const mySkills = new Skill();
console.log([...mySkills]);
for (let skill of mySkills) {
    console.log(`My skill is ${skill}`);
}

'angular'.search('ng') // 4

// pseudo code for String class
class String {
    constructor(value) {
        this.value = value;
    }
    search(obj) {
        obj[Symbol.search](this.value);
    }
}
class RegExp {
    constructor(value) {
        this.value = value;
    }
    [Symbol.search](string) {
        return string.indexOf(this.value);
    }
}

class Article {
    constructor(tag) {
        this.tag = tag;
    }
    [Symbol.search](string) {
        return string.indexOf(this.tag) >= 0 ? 'Found' : 'Not_Found';
    }
}
var article = new Article('Angular');
console.log('Angular7'.search(article)); // Found
console.log('重温ES6'.search(article)); // Not_Found