JavaScript

const target = { name: 'Sarah', age: 25 };
const handler = {
  get(obj, prop) {
    if (prop === 'toString') {
      return () => `Person: ${obj.name}`;
    }
    return Reflect.get(obj, prop);
  },
  has(obj, prop) {
    return prop !== 'age' && Reflect.has(obj, prop);
  }
};
const proxy = new Proxy(target, handler);
console.log(proxy.name);
console.log('age' in proxy);
console.log(proxy.toString());

class StateMachine {
  constructor() {
    this.state = 'idle';
    this.transitions = {
      idle: { start: 'running', reset: 'idle' },
      running: { pause: 'paused', stop: 'stopped' },
      paused: { resume: 'running', stop: 'stopped' },
      stopped: { reset: 'idle' }
    };
  }
  
  transition(action) {
    const nextState = this.transitions[this.state]?.[action];
    if (nextState) {
      this.state = nextState;
      return true;
    }
    return false;
  }
}

const sm = new StateMachine();
console.log(sm.transition('start'));
console.log(sm.state);
console.log(sm.transition('reset'));
console.log(sm.state);

class ValidationError extends Error {
  constructor(message, code) {
    super(message);
    this.name = 'ValidationError';
    this.code = code;
  }
}

function processData(data) {
  try {
    if (!data) throw new ValidationError('Data required', 'MISSING_DATA');
    return data.toUpperCase();
  } catch (error) {
    if (error instanceof ValidationError) {
      console.log(`${error.name}: ${error.message} (${error.code})`);
    } else {
      console.log(`Unexpected error: ${error.message}`);
    }
  }
}

processData(null);
processData(42);

class CustomError extends Error {
  constructor(message, code) {
    super(message);
    this.name = 'CustomError';
    this.code = code;
  }
}

try {
  throw new CustomError('Something went wrong', 500);
} catch (error) {
  console.log(error instanceof Error);
  console.log(error instanceof CustomError);
  console.log(error.name);
  console.log(typeof error.code);
}

let cache = new Map();
let obj = { name: 'Emma', age: 28 };

function memoize(fn) {
  return function(...args) {
    const key = JSON.stringify(args);
    if (cache.has(key)) {
      console.log('Cache hit');
      return cache.get(key);
    }
    const result = fn.apply(this, args);
    cache.set(key, result);
    return result;
  };
}

const process = memoize((data) => {
  console.log('Processing:', data.name);
  return data.age * 2;
});

process(obj);
process(obj);
obj.age = 30;
process(obj);
console.log('Cache size:', cache.size);

const pattern = /(?<=\d)(?=(?:\d{3})+(?!\d))/g;
const str = '1234567890';
const result = str.replace(pattern, ',');
console.log(result);

const regex = /^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)[a-zA-Z\d]{8,}$/;
const tests = ['Password1', 'password', 'PASSWORD1', 'Pass1'];
tests.forEach(test => {
  console.log(`${test}: ${regex.test(test)}`);
});

const capture = /([a-z]+)(\d+)/g;
const text = 'item123 product456';
const matches = [...text.matchAll(capture)];
console.log(matches[1][1]);

const sym1 = Symbol('test');
const sym2 = Symbol('test');
const obj = {
  [sym1]: 'value1',
  [sym2]: 'value2',
  regular: 'value3'
};

const sym3 = Symbol.for('global');
const sym4 = Symbol.for('global');

console.log(sym1 === sym2);
console.log(sym3 === sym4);
console.log(Object.keys(obj).length);
console.log(Object.getOwnPropertySymbols(obj).length);

const cache = new Map();

function createHandler(id) {
  const data = { id, value: new Array(1000).fill(id) };
  cache.set(id, data);
  
  return function(action) {
    if (action === 'get') return data;
    if (action === 'clear') cache.delete(id);
  };
}

const handler1 = createHandler('user-1');
const handler2 = createHandler('user-2');

handler1('clear');
console.log(cache.size);
console.log(handler1('get').id);