JavaScript

const target = { name: 'Sarah', age: 25 };

const handler = {
  get(obj, prop) {
    if (prop in obj) {
      return obj[prop];
    }
    return `Property '${prop}' not found`;
  },
  set(obj, prop, value) {
    obj[prop] = value.toString().toUpperCase();
    return true;
  }
};

const proxy = new Proxy(target, handler);
proxy.city = 'boston';
console.log(proxy.name);
console.log(proxy.city);
console.log(proxy.country);

class Logger {
  constructor(prefix) {
    this.prefix = prefix;
  }
  log(message) {
    console.log(`${this.prefix}: ${message}`);
  }
}

class Database {
  constructor(logger) {
    this.logger = logger;
  }
  save(data) {
    this.logger.log(`Saving ${data}`);
    return `${data}_saved`;
  }
}

const logger = new Logger('DB');
const db = new Database(logger);
const result = db.save('user');
console.log(result);

const Flyable = {
  fly() { return `${this.name} is flying`; }
};

const Swimmable = {
  swim() { return `${this.name} is swimming`; }
};

function createDuck(name) {
  return Object.assign({ name }, Flyable, Swimmable);
}

const duck = createDuck('Quackers');
console.log(duck.fly());
console.log(duck.swim());
console.log(Object.getOwnPropertyNames(duck));

const compose = (...fns) => x => fns.reduceRight((acc, fn) => fn(acc), x);

const addPrefix = str => `prefix_${str}`;
const capitalize = str => str.charAt(0).toUpperCase() + str.slice(1);
const addSuffix = str => `${str}_suffix`;

const process = compose(addSuffix, capitalize, addPrefix);

console.log(process('data'));

class VideoCall {
  constructor() {
    this.pc = { iceConnectionState: 'new' };
    this.streams = [];
  }
  
  async connect() {
    this.pc.iceConnectionState = 'checking';
    await Promise.resolve();
    this.pc.iceConnectionState = 'connected';
    this.streams.push('remote-video');
    return this.pc.iceConnectionState;
  }
  
  handleConnectionChange() {
    const states = ['new', 'checking', 'connected'];
    return states.map(state => {
      return this.pc.iceConnectionState === state ? `Status: ${state}` : null;
    }).filter(Boolean);
  }
}

const call = new VideoCall();
call.connect().then(() => {
  console.log(call.handleConnectionChange());
});

class ChatServer {
  constructor() {
    this.clients = new Set();
    this.messageLog = [];
  }
  
  connect(client) {
    this.clients.add(client);
    return () => this.clients.delete(client);
  }
  
  broadcast(message, sender) {
    this.messageLog.push(message);
    this.clients.forEach(client => {
      if (client !== sender) {
        client.receive(message);
      }
    });
  }
}

const server = new ChatServer();
const david = { name: 'David', receive: msg => console.log(`David got: ${msg}`) };
const sarah = { name: 'Sarah', receive: msg => console.log(`Sarah got: ${msg}`) };
const emma = { name: 'Emma', receive: msg => console.log(`Emma got: ${msg}`) };

const disconnectDavid = server.connect(david);
server.connect(sarah);
server.connect(emma);
server.broadcast('Hello everyone!', david);
disconnectDavid();
server.broadcast('Is David still here?', sarah);

console.log(server.clients.size);

function greet(name) {
  return `Hello, ${name}!`;
}

function highlight(strings, ...values) {
  return strings.reduce((result, str, i) => {
    return result + str + (values[i] ? `<em>${values[i]}</em>` : '');
  }, '');
}

const user = 'Sarah';
const status = 'online';

console.log(highlight`User ${user} is currently ${status}.`);

async function processValues() {
  try {
    console.log('Start');
    const a = await Promise.resolve('First');
    console.log(a);
    const b = await Promise.reject('Error');
    console.log(b);
    return 'Done';
  } catch (err) {
    console.log(err);
    return 'Recovered';
  } finally {
    console.log('Finally');
  }
}

processValues().then(result => console.log(result));