LearnES6part3 created by alexspark - https://repl.it/@alexspark/LearnES6part3

LearnES6part3.js

// Set data structure
let s = new Set();
s.add(1);
s.add(2);
s.add(3);
console.log(s.size);

console.log(s.has(2), s.has(10));

for (let k of s.values()) {
  console.log(k);
}

// Map data structure
let m = new Map();
m.set('hello', 42);
m.set('alex', 29);
m.set('eunice', 30);
console.log(m);

for (let [k, v] of m.entries()) {
  console.log(`key: ${k}, value: ${v}`)
}

let isMarked     = new WeakSet()
let attachedData = new WeakMap()

// weakSet & weakMap
class Node {
    constructor (id)   { this.id = id                  }
    mark        ()     { isMarked.add(this)            }
    unmark      ()     { isMarked.delete(this)         }
    marked      ()     { return isMarked.has(this)     }
    set data    (data) { attachedData.set(this, data)  }
    get data    ()     { return attachedData.get(this) }
}

let foo = new Node("foo")

JSON.stringify(foo) === '{"id":"foo"}'
foo.mark()
foo.data = "bar"
foo.data === "bar"
JSON.stringify(foo) === '{"id":"foo"}'

isMarked.has(foo)     === true
attachedData.has(foo) === true
foo = null  /* remove only reference to foo */
attachedData.has(foo) === false
isMarked.has(foo)     === false

// type arrays
class Example {
    constructor (buffer = new ArrayBuffer(24)) {
        this.buffer = buffer
    }
    set buffer (buffer) {
        this._buffer    = buffer
        this._id        = new Uint32Array (this._buffer,  0,  1)
        this._username  = new Uint8Array  (this._buffer,  4, 16)
        this._amountDue = new Float32Array(this._buffer, 20,  1)
    }
    get buffer ()     { return this._buffer       }
    set id (v)        { this._id[0] = v           }
    get id ()         { return this._id[0]        }
    set username (v)  { this._username[0] = v     }
    get username ()   { return this._username[0]  }
    set amountDue (v) { this._amountDue[0] = v    }
    get amountDue ()  { return this._amountDue[0] }
}

let example = new Example()
example.id = 7
example.username = "John Doe"
example.amountDue = 42.0

// New built-in methods
Object.assign({}, { a: 'b' }, { a: 'c' });
[1, 2, 3].find(x => x >= 3);
[1, 2, 3].findIndex(x => x >= 3)
'*'.repeat(16);
'hello_world'.startsWith('ello', 1);
'hello_world'.endsWith('world', 11);
Number.isNaN(24);
Number.isFinite(123);
Number.isFinite(NaN);
Number.isFinite(Infinity);
Number.isSafeInteger(42) === true
Number.isSafeInteger(9007199254740992) === false
Number.EPSILON;
console.log(`
all numbers are double-precision 64-bit IEEE 754 values
.2 + .1 = ${0.2 + 0.1}
`);
console.log(0.1 + 0.2 === 0.3) // false
console.log(Math.abs((0.1 + 0.2) - 0.3) < Number.EPSILON);

Math.trunc(42.7);
Math.trunc(0.1);
Math.sign(7);
Math.sign(-2);
Math.sign(0);
Math.sign(-0);
Math.sign(NaN);

// Promises
const promFactory = {
  *[Symbol.iterator]() {
    let count = 0;
    for(;;) {
      count += 1;
      yield new Promise((resolve, reject) => {
        if (count < 100) {
          resolve(count);
        } else {
          reject(`count is ${count}. count exceeded 100!`);
        }
      });
    }
  }
}

// for (p of promFactory) {
//   let ex = false;
//   p.then((v) => {
//     console.log(`value: ${v}`);
//   }).catch(err => {
//     console.log(`error: ${err}`);
//     ex = true;
//   });
//   if (ex) break;
// }

// proxying
let target = {
    foo: "Welcome, foo"
}
let proxy = new Proxy(target, {
    get (receiver, name) {
        return name in receiver ? receiver[name] : `Hello, ${name}`
    }
})
proxy.foo   === "Welcome, foo"
proxy.world === "Hello, world"