Что и как в ES6: хитрости, лучшие практики и примеры. Часть вторая. Мэпы, слабые мэпы, обещания, генераторы, async / await, геттеры / сеттеры, символы

			var map = new Object();
map[key1] = 'value1';
map[key2] = 'value2';
		

			> getOwnProperty({ hasOwnProperty: 'Hah, overwritten'}, 'Pwned');
> TypeError: Property 'hasOwnProperty' is not a function
		

			let map = new Map();
> map.set('name', 'david');
> map.get('name'); // david
> map.has('name'); // true
		

			let map = new Map([
    ['name', 'david'],
    [true, 'false'],
    [1, 'one'],
    [{}, 'object'],
    [function () {}, 'function']
]);

for (let key of map.keys()) {
    console.log(typeof key);
    // > string, boolean, number, object, function
}
		

			for (let [key, value] of map.entries()) {
    console.log(key, value);
}
		

			class Person {
    constructor(age) {
        this._age = age;
    }

    _incrementAge() {
        this._age += 1;
    }
}
		

			let _age = new WeakMap();
class Person {
    constructor(age) {
        _age.set(this, age);
    }

    incrementAge() {
        let age = _age.get(this) + 1;
        _age.set(this, age);
        if (age > 50) {
            console.log('Midlife crisis');
        }
    }
}
		

			> const person = new Person(50);
> person.incrementAge(); // 'Midlife crisis'
> Reflect.ownKeys(person); // []
		

			let map = new WeakMap();
let el  = document.getElementById('someElement');

// Store a weak reference to the element with a key
map.set(el, 'reference');

// Access the value of the element
let value = map.get(el); // 'reference'

// Remove the reference
el.parentNode.removeChild(el);
el = null;

// map is empty, since the element is destroyed
		

			func1(function (value1) {
    func2(value1, function (value2) {
        func3(value2, function (value3) {
            func4(value3, function (value4) {
                func5(value4, function (value5) {
                    // Do something with value 5
                });
            });
        });
    });
});
		

			func1(value1)
    .then(func2)
    .then(func3)
    .then(func4)
    .then(func5, value5 => {
        // Do something with value 5
    });
		

			new Promise((resolve, reject) =>
    reject(new Error('Failed to fulfill Promise')))
        .catch(reason => console.log(reason));
		

			var request = require('request');

return new Promise((resolve, reject) => {
  request.get(url, (error, response, body) => {
    if (body) {
        resolve(JSON.parse(body));
      } else {
        resolve({});
      }
  });
});
		

			let urls = [
  '/api/commits',
  '/api/issues/opened',
  '/api/issues/assigned',
  '/api/issues/completed',
  '/api/issues/comments',
  '/api/pullrequests'
];

let promises = urls.map((url) => {
  return new Promise((resolve, reject) => {
    $.ajax({ url: url })
      .done((data) => {
        resolve(data);
      });
  });
});

Promise.all(promises)
  .then((results) => {
    // Do something with results of all our promises
 });
		

			function* sillyGenerator() {
    yield 1;
    yield 2;
    yield 3;
    yield 4;
}

var generator = sillyGenerator();
> console.log(generator.next()); // { value: 1, done: false }
> console.log(generator.next()); // { value: 2, done: false }
> console.log(generator.next()); // { value: 3, done: false }
> console.log(generator.next()); // { value: 4, done: false }
		

			// Hiding asynchronousity with Generators

function request(url) {
    getJSON(url, function(response) {
        generator.next(response);
    });
}
		

			function* getData() {
    var entry1 = yield request('http://some_api/item1');
    var data1  = JSON.parse(entry1);
    var entry2 = yield request('http://some_api/item2');
    var data2  = JSON.parse(entry2);
}
		

			function request(url) {
    return new Promise((resolve, reject) => {
        getJSON(url, resolve);
    });
}
		

			function iterateGenerator(gen) {
    var generator = gen();
    (function iterate(val) {
        var ret = generator.next();
        if(!ret.done) {
            ret.value.then(iterate);
        }
    })();
}
		

			iterateGenerator(function* getData() {
    var entry1 = yield request('http://some_api/item1');
    var data1  = JSON.parse(entry1);
    var entry2 = yield request('http://some_api/item2');
    var data2  = JSON.parse(entry2);
});
		

			var request = require('request');

function getJSON(url) {
  return new Promise(function(resolve, reject) {
    request(url, function(error, response, body) {
      resolve(body);
    });
  });
}

async function main() {
  var data = await getJSON();
  console.log(data); // NOT undefined!
}

main();
		

			class Employee {

    constructor(name) {
        this._name = name;
    }

    get name() {
      if(this._name) {
        return 'Mr. ' + this._name.toUpperCase();  
      } else {
        return undefined;
      }  
    }

    set name(newName) {
      if (newName == this._name) {
        console.log('I already have this name.');
      } else if (newName) {
        this._name = newName;
      } else {
        return false;
      }
    }
}

var emp = new Employee("James Bond");

// uses the get method in the background
if (emp.name) {
  console.log(emp.name);  // Mr. JAMES BOND
}

// uses the setter in the background
emp.name = "Bond 007";
console.log(emp.name);  // Mr. BOND 007
		

			var person = {
  firstName: 'James',
  lastName: 'Bond',
  get fullName() {
      console.log('Getting FullName');
      return this.firstName + ' ' + this.lastName;
  },
  set fullName (name) {
      console.log('Setting FullName');
      var words = name.toString().split(' ');
      this.firstName = words[0] || '';
      this.lastName = words[1] || '';
  }
}

person.fullName; // James Bond
person.fullName = 'Bond 007';
person.fullName; // Bond 007
		

			const refreshComponent = Symbol();

React.Component.prototype[refreshComponent] = () => {
    // do something
}
		

			Symbol('foo') === Symbol('foo') // false
Symbol.for('foo') === Symbol('foo') // false
Symbol.for('foo') === Symbol.for('foo') // true
		

			function reader(obj) {
    const specialRead = Symbol.for('specialRead');
    if (obj[specialRead]) {
        const reader = obj[specialRead]();
        // do something with reader
    } else {
        throw new TypeError('object cannot be read');
    }
}
		

			const specialRead = Symbol.for('specialRead');

class SomeReadableType {
    [specialRead]() {
        const reader = createSomeReaderFrom(this);
        return reader;
    }
}