JavaScript

CHALLENGE

class Base {}
class Derived extends Base {
  static [Symbol.hasInstance](instance) {
    return false;
  }
}
const d = new Derived();
console.log(d instanceof Derived, d instanceof Base, Object.getPrototypeOf(Derived) === Base);

CHALLENGE

function makeFns() {
  const result = [];
  for (var i = 0; i < 3; i++) {
    let j = i;
    result.push(() => i + j);
  }
  return result;
}
const fns = makeFns();
console.log(fns.map(f => f()).join(','));

CHALLENGE

const arr = [1, [2, 3], { a: 4 }];
const copy = [...arr];
copy[1].push(99);
copy[2].a = 100;
arr[0] = 999;

console.log(arr[0], arr[1], arr[2].a, copy[0]);

CHALLENGE

const a = Math.max();
const b = Math.min();
const c = 0.1 + 0.2 === 0.3;
const d = Math.max(1, NaN, 3);
const e = [1, 2, 3].reduce((sum, n) => sum + n, 0) / 3;
const f = Number.isInteger(5.0);
console.log(a, b, c, d, e, f);

CHALLENGE

function makeCounters() {
  const counters = [];
  for (var i = 0; i < 3; i++) {
    let j = i;
    counters.push(() => `${i}-${j}`);
  }
  return counters;
}
const [a, b, c] = makeCounters();
console.log(a(), b(), c());
export {};

CHALLENGE

class EventBus {
  #listeners = new Map();
  on(event, fn) {
    if (!this.#listeners.has(event)) this.#listeners.set(event, new Set());
    this.#listeners.get(event).add(fn);
    return () => this.#listeners.get(event).delete(fn);
  }
  emit(event, payload) {
    this.#listeners.get(event)?.forEach(fn => fn(payload));
  }
}
const bus = new EventBus();
const log = [];
const unsub = bus.on('data', v => log.push(`A:${v}`));
bus.on('data', v => log.push(`B:${v}`));
bus.emit('data', 1);
unsub();
bus.on('data', v => log.push(`C:${v}`));
bus.emit('data', 2);
console.log(log.join(','));

CHALLENGE

function Person(name) {
  if (!(this instanceof Person)) {
    return new Person(name);
  }
  this.name = name;
}

Person.prototype.greet = function () {
  return `Hi ${this.name}`;
};

function Widget(id) {
  this.id = id;
  return { id: id * 2 };
}

Widget.prototype.getId = function () {
  return this.id;
};

const p1 = Person('Zed');
const p2 = new Person('Nova');
const w = new Widget(5);

console.log(p1.greet(), p2.greet(), w.id, w.getId);

😮 DeepSeek Harness: DeepSeek's New Node-Powered Agent Harness Today, the popular Chinese model lab unveiled its own Claude Code-alike and already racked up 30k stars. It's not a typical CLI harness, though, but runs through a web UI. Curiously, everything is a plugin, built atop Cordis, an existing Node plugin system whose author DeepSeek has hired. GitHub repo. DeepSeek

CHALLENGE

const log = [];
Promise.resolve(1)
  .then(v => { log.push('a'+v); return v+1; })
  .then(v => { throw new Error('e'+v); })
  .catch(e => { log.push(e.message); return 10; })
  .then(v => { log.push('b'+v); });

Promise.resolve()
  .then(() => log.push('c'))
  .then(() => log.push('d'));

setTimeout(() => console.log(log.join(',')), 0);

❓ TermDOM: Build Terminal UIs with HTML, CSS and the DOM Like the look of Ink but don't like React? TermDOM implements a DOM, cascade and layout engine that paints to the terminal, so you can write a TUI with HTML and CSS. Pure JS, no native or WASM dependencies, and the official TodoMVC runs with only a stylesheet swap. Early days, but I like the idea! Brian Kim

CHALLENGE

function curry(fn) {
  return function curried(...args) {
    if (args.length >= fn.length) {
      return fn.apply(this, args);
    }
    return (...more) => curried.apply(this, args.concat(more));
  };
}

function add(a, b, c = 10) {
  return a + b + c;
}

const curriedAdd = curry(add);
console.log(`${curriedAdd(1)(2)} ${curriedAdd(1,2,3)} ${curriedAdd(4)(5,6)}`);