JavaScript

const partial = (fn, ...presetArgs) => {
  return function partiallyApplied(...laterArgs) {
    return fn(...presetArgs, ...laterArgs);
  };
};

const calculateTax = (taxRate, discount, price) => {
  const discounted = price - (price * discount) / 100;
  return parseFloat((discounted + (discounted * taxRate) / 100).toFixed(2));
};

const withVAT = partial(calculateTax, 20);
const withVATandDiscount = partial(withVAT, 15);

console.log(withVAT(0, 100));
console.log(withVATandDiscount(200));
console.log(partial(calculateTax, 10, 5)(50));

const counter = (function () {
  let count = 0;
  const history = [];

  return {
    increment(step = 1) {
      count += step;
      history.push(count);
      return this;
    },
    decrement(step = 1) {
      count -= step;
      history.push(count);
      return this;
    },
    getHistory: () => history,
    getCount: () => count,
  };
})();

counter.increment(5).increment(3).decrement(2);

console.log(counter.getCount(), counter.getHistory().join(" -> "));

const user = {
  profile: {
    name: "Marcus",
    score: 0,
    nickname: null,
    bio: undefined,
  },
};

user.profile.nickname ??= "Anonymous";
user.profile.bio ??= "No bio provided";
user.profile.score ??= 100;
user.profile.rank ??= "Beginner";

const { name, score, nickname, bio, rank } = user.profile;
console.log(`${name} | ${score} | ${nickname} | ${bio} | ${rank}`);

class Animal {
  #sound;

  constructor(name, sound) {
    this.name = name;
    this.#sound = sound;
  }

  speak() {
    return `${this.name} says ${this.#sound}`;
  }

  static create(name, sound) {
    return new this(name, sound);
  }
}

class Dog extends Animal {
  #tricks = [];

  constructor(name) {
    super(name, "woof");
  }

  learn(trick) {
    this.#tricks.push(trick);
    return this;
  }

  perform() {
    return `${this.name} performs: ${this.#tricks.join(", ")}`;
  }
}

const rex = Dog.create("Rex", "bark");
rex.learn("sit").learn("shake").learn("roll over");

console.log(rex.speak());
console.log(rex.perform());
console.log(rex instanceof Animal);
console.log(rex instanceof Dog);

const values = [0.1 + 0.2, NaN, Infinity, -0, 42.6789];

const results = values.map((v, i) => {
  if (i === 0) return Number.isInteger(v) + " " + v.toFixed(1);
  if (i === 1) return Number.isFinite(v) + " " + Number.isNaN(v);
  if (i === 2) return Number.isFinite(v) + " " + isFinite(v);
  if (i === 3) return Object.is(v, -0) + " " + (v === 0);
  if (i === 4) return v.toPrecision(4) + " " + Math.trunc(v);
});

results.forEach(r => console.log(r));

const prefix = 'user';
const id = 42;
const role = 'admin';

const capitalize = str => str.charAt(0).toUpperCase() + str.slice(1);

const profile = {
  [`${prefix}_${id}`]: 'Marcus',
  [Symbol.iterator]: null,
  [`get${capitalize(role)}`]: () => 'full-access',
  [`${prefix}Count`]: 3,
};

const dynamicKey = `${prefix}_${id}`;

console.log(
  profile[dynamicKey],
  profile[`get${capitalize(role)}`](),
  profile[`${prefix}Count`] * id,
  Object.keys(profile).length
);