JavaScript

const obj = {
  name: 'Calculator',
  values: [1, 2, 3, 4, 5],
  
  regular: function() {
    return this.values.filter(function(x) {
      return x > 2;
    }).length;
  },
  
  arrow: function() {
    return this.values.filter(x => x > 2).length;
  },
  
  broken: () => {
    return this.values.filter(x => x > 2).length;
  }
};

console.log(obj.regular());
console.log(obj.arrow());
try {
  console.log(obj.broken());
} catch(e) {
  console.log('Error');
}

const user = { name: 'Sarah', age: 0 };
const config = { theme: '', debug: false };

const result1 = user.age || 25;
const result2 = user.name && user.name.toUpperCase();
const result3 = config.theme || 'light';
const result4 = config.debug && console.log('Debug mode') || 'disabled';

console.log(result1);
console.log(result2);
console.log(result3);
console.log(result4);

const pipe = (...fns) => (value) => fns.reduce((acc, fn) => fn(acc), value);

const add = (x) => (y) => x + y;
const multiply = (x) => (y) => x * y;
const subtract = (x) => (y) => y - x;

const transform = pipe(
  add(5),
  multiply(3),
  subtract(2)
);

console.log(transform(4));
console.log(transform(0));

class Counter {
  #value = 0;
  
  get count() {
    console.log('Getting count');
    return this.#value;
  }
  
  set count(val) {
    console.log('Setting count to', val);
    this.#value = Math.max(0, val);
  }
}

const counter = new Counter();
counter.count = -5;
console.log(counter.count);

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

const name = 'Sarah';
const score = 95;
const message = highlight`Hello ${name}, your score is ${score}!`;
console.log(message);

async function processData() {
  const promise1 = Promise.resolve('first');
  const promise2 = Promise.reject('error');
  const promise3 = Promise.resolve('third');
  
  try {
    const result = await Promise.allSettled([promise1, promise2, promise3]);
    console.log(result[0].status);
    console.log(result[1].reason);
    console.log(result[2].value);
  } catch (error) {
    console.log('caught:', error);
  }
}

processData();

const data = [
  { type: 'income', amount: 1000, category: 'salary' },
  { type: 'expense', amount: 200, category: 'food' },
  { type: 'income', amount: 500, category: 'freelance' },
  { type: 'expense', amount: 150, category: 'transport' }
];

const result = data.reduce((acc, item) => {
  const key = item.type;
  acc[key] = (acc[key] || 0) + item.amount;
  return acc;
}, {});

console.log(result.income - result.expense);

console.log(typeof myVar);
console.log(typeof myFunc);
console.log(typeof myArrow);

var myVar = 'initialized';

function myFunc() {
  return 'function declaration';
}

var myArrow = () => 'arrow function';

console.log(typeof myVar);
console.log(typeof myFunc);
console.log(typeof myArrow);

const config = { api: 'v1', timeout: 5000 };
Object.seal(config);

const settings = { theme: 'dark', lang: 'en' };
Object.freeze(settings);

config.api = 'v2';
config.retries = 3;
settings.theme = 'light';
settings.debug = true;

console.log(config.api);
console.log(config.retries);
console.log(settings.theme);
console.log(settings.debug);