JavaScript

21 January 2026 07:32

CHALLENGE

function mysteryFunction() {
  console.log(x);
  console.log(y);
  console.log(z);
  
  var x = 'declared';
  let y = 'block-scoped';
  const z = 'constant';
  
  console.log(x);
  console.log(y);
  console.log(z);
}

mysteryFunction();

JavaScript

20 January 2026 07:31

CHALLENGE

const obj = {
  value: 42,
  [Symbol.toPrimitive](hint) {
    if (hint === 'number') return this.value * 2;
    if (hint === 'string') return `Value: ${this.value}`;
    return this.value + 10;
  }
};

console.log(+obj);
console.log(`${obj}`);
console.log(obj + 5);
console.log(Number(obj));

JavaScript

18 January 2026 16:35

🔥 The unhinged world of tech in 2026...

Fireship

JavaScript

16 January 2026 19:07

✌️ Date is out, Temporal is in

Temporal is the Date system we always wanted in JavaScript. It's extremely close to being available so Mat Marquis thought it would be a good idea to explain exactly what is better about this new JavaScript date system.

Mat “Wilto” Marquis

JavaScript

15 January 2026 07:30

CHALLENGE

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);

JavaScript

14 January 2026 07:30

CHALLENGE

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));

JavaScript

12 January 2026 07:32

CHALLENGE

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);

JavaScript

10 January 2026 07:50

CHALLENGE

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);

JavaScript

09 January 2026 07:34

CHALLENGE

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);

JavaScript

08 January 2026 07:31

CHALLENGE

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);

JavaScript

07 January 2026 07:33

CHALLENGE

const obj = { a: 1, b: 2, c: 3 };
const entries = Object.entries(obj);
const keys = Object.keys(obj);
const values = Object.values(obj);

const result = {
  entriesLength: entries.length,
  keysJoined: keys.join('-'),
  valuesSum: values.reduce((sum, val) => sum + val, 0),
  firstEntry: entries[0]
};

console.log(result.entriesLength);
console.log(result.keysJoined);
console.log(result.valuesSum);
console.log(result.firstEntry);

JavaScript

05 January 2026 07:33

CHALLENGE

const user = {
  name: 'Sarah',
  age: 28,
  getName() {
    return this.name;
  }
};

const { getName } = user;
const boundGetName = user.getName.bind(user);

console.log(getName());
console.log(boundGetName());
console.log(user.getName());

JavaScript

03 January 2026 07:34

CHALLENGE

async function fetchData() {
  return Promise.resolve('data');
}

async function processData() {
  console.log('start');
  const result = fetchData();
  console.log(typeof result);
  const data = await fetchData();
  console.log(typeof data);
  console.log('end');
}

processData();

JavaScript

31 December 2025 19:37

Happy New Year! 🎄 🍾

Wishing you fewer meetings, more merges, and no Friday deploys. 😆

@JavaScript Telegram Newsletter Team

JavaScript

29 December 2025 01:39

Runtime of the year 🤔

JavaScript

20 January 2026 14:39

🤟 Node.js v25.4.0 is out!

• require(esm) now stable and a new CLI flag: --require-module
• Module compile cache now stable
• http.setGlobalProxyFromEnv() added
• Multiple APIs promoted to stable (heapsnapshot, build snapshot, `v8.queryObjects`)
• Root CAs updated to NSS 3.117
• Several semver-minor improvements across events, module, stream, process, util

Rafael Gonzaga

JavaScript

19 January 2026 14:53

A year ago, developer Dimitri Mitropoulos got Doom to run inside TypeScript's type system. Now, he's joined Dillon Mulroy (above) 😉 to walk through the entirety of how it works (in a mere six hours!)

JavaScript

18 January 2026 13:48

CHALLENGE

const numbers = [1, 2, 3, 4, 5, 6];

const result = numbers
  .map(x => x * 2)
  .filter(x => x > 6)
  .reduce((acc, x) => {
    console.log(`Processing ${x}, acc: ${acc}`);
    return acc + x;
  }, 0);

console.log(`Final result: ${result}`);

JavaScript

16 January 2026 07:30

CHALLENGE

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');
}

JavaScript

14 January 2026 14:19

✌️ Web Dependencies are Broken; Can We Fix Them?

Lea, who has worked at the heart of Web Standards for years, delivers a compelling (and educational) call to action about a problem every JavaScript developer has encountered: why is managing dependencies and introducing them into code so unnecessarily messy and what could we do about it?

Lea Verou

JavaScript

13 January 2026 07:33

CHALLENGE

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);

JavaScript

11 January 2026 07:31

CHALLENGE

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();

JavaScript

09 January 2026 13:39

👀 For years, Mozilla, Apple, and the CSS Working Group have been working to bring "masonry" layouts (as above) natively to CSS. The concept is now called CSS Grid Lanes and here's how it works. You can already try it out in Safari Technology Preview 234.

JavaScript

08 January 2026 13:04

✌️🌲📸🟠🔵 Schedule-X 3.6: A Material Design Calendar and Date Picker

Available in the form of React/Preact, Vue, Svelte, Angular, or plain JS components. Open source but with a premium version with extra features. GitHub repo.

Tom Österlund

JavaScript

07 January 2026 10:02

😮 The 2025 JavaScript Rising Stars

At the start of each year, Michael rounds up the projects in the JavaScript ecosystem that gained the most popularity on GitHub in the prior year. After a two-year run of topping the chart, shadcn/ui has been pushed down to #3 by n8n and React Bits. This is a fantastic roundup, now in its tenth(!) year, and features commentary from a few industry experts too.

Michael Rambeau et al.

JavaScript

06 January 2026 07:30

CHALLENGE

function* outer() {
  yield 1;
  yield* inner();
  yield 4;
}

function* inner() {
  yield 2;
  yield 3;
}

const gen = outer();
console.log(gen.next().value);
console.log(gen.next().value);
console.log(gen.next().value);
console.log(gen.next().value);

JavaScript

04 January 2026 07:32

CHALLENGE

function* fibonacci() {
  let a = 0, b = 1;
  yield a;
  yield b;
  while (true) {
    let next = a + b;
    yield next;
    a = b;
    b = next;
  }
}

const gen = fibonacci();
const results = [];
for (let i = 0; i < 6; i++) {
  results.push(gen.next().value);
}
console.log(results.join(','));

JavaScript

02 January 2026 07:33

CHALLENGE

class Vehicle {
  #engine = 'V6';
  static count = 0;
  
  constructor(type) {
    this.type = type;
    Vehicle.count++;
  }
  
  static getCount() {
    return this.count;
  }
  
  get info() {
    return `${this.type} with ${this.#engine}`;
  }
}

class Car extends Vehicle {
  static count = 0;
  
  constructor(brand) {
    super('car');
    this.brand = brand;
    Car.count++;
  }
}

const tesla = new Car('Tesla');
const ford = new Car('Ford');
console.log(Vehicle.getCount());
console.log(Car.getCount());
console.log(tesla.info);

JavaScript

31 December 2025 18:01

Framework/lib of the year 🤔

JavaScript

28 December 2025 07:34

CHALLENGE

function createCounter() {
  let count = 0;
  return function(increment = 1) {
    count += increment;
    return count;
  };
}

const counter1 = createCounter();
const counter2 = createCounter();

console.log(counter1());
console.log(counter1(5));
console.log(counter2(3));
console.log(counter1());
console.log(counter2());