JavaScript

const arr = [1, 2, 3];
const obj = { valueOf: () => 4, toString: () => '5' };
const result1 = arr + obj;
const result2 = +obj;
const result3 = String(obj);
const result4 = obj == 4;
const result5 = obj === 4;
console.log(`${result1}|${result2}|${result3}|${result4}|${result5}`);
const weird = [] + [] + 'hello';
const weirder = [] + {} + [];
const weirdest = {} + [] + {};
console.log(`${weird}|${weirder}|${weirdest}`);
const final = !!'0' + !!'' + !!null + !!undefined;
console.log(final);

class SimpleObservable {
  constructor(subscribeFn) {
    this.subscribeFn = subscribeFn;
  }
  
  subscribe(observer) {
    return this.subscribeFn(observer);
  }
}

const obs = new SimpleObservable(observer => {
  observer.next('first');
  observer.next('second');
  observer.complete();
});

const results = [];
obs.subscribe({
  next: val => results.push(val),
  complete: () => results.push('done')
});

console.log(results.join('-'));

const original = {
  name: 'Sarah',
  hobbies: ['reading', 'coding'],
  address: { city: 'Portland', zip: 97201 }
};

const shallow = { ...original };
const deep = JSON.parse(JSON.stringify(original));

shallow.name = 'Emma';
shallow.hobbies.push('hiking');
shallow.address.city = 'Seattle';

deep.hobbies.push('swimming');
deep.address.zip = 98101;

console.log(original.hobbies.length, original.address.city);

const user = {
  profile: {
    settings: {
      theme: 'dark',
      notifications: null
    }
  }
};

const result1 = user?.profile?.settings?.theme;
const result2 = user?.profile?.settings?.notifications?.email;
const result3 = user?.profile?.preferences?.language ?? 'en';
const result4 = user?.profile?.settings?.notifications?.push?.('test');

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

console.log(typeof myFunction);
console.log(typeof myVar);
console.log(typeof myLet);
console.log(typeof myConst);

var myVar = 'hello';
let myLet = 'world';
const myConst = 'test';

function myFunction() {
  return 'hoisted';
}

console.log(myFunction());
console.log(myVar);