What is the output?
Anonymous Quiz
49%
[LOG]: HELLO WORLD
29%
HELLO WORLD
14%
hello world
8%
[LOG]: hello world
❤2
CHALLENGE
let obj1 = { name: 'Sarah' };
let obj2 = { name: 'Mike' };
obj1.ref = obj2;
obj2.ref = obj1;
let weakRef = new WeakRef(obj1);
let registry = new FinalizationRegistry((value) => {
console.log(`Cleanup: ${value}`);
});
registry.register(obj1, 'obj1-cleaned');
obj1 = null;
obj2 = null;
console.log(weakRef.deref()?.name || 'undefined');
console.log('Script completed');What is the output?
Anonymous Quiz
30%
Sarah Script completed
41%
undefined Script completed Cleanup: obj1-cleaned
20%
Sarah Script completed Cleanup: obj1-cleaned
9%
undefined Script completed
🔥4🤔3❤1
CHALLENGE
class EventEmitter {
constructor() {
this.events = {};
}
on(event, callback) {
this.events[event] = this.events[event] || [];
this.events[event].push(callback);
return this;
}
emit(event, data) {
if (this.events[event]) {
this.events[event].forEach(cb => cb(data));
}
return this;
}
}
const emitter = new EventEmitter();
emitter.on('test', x => console.log(x * 2))
.on('test', x => console.log(x + 5))
.emit('test', 10);❤7
Back in May 1995, a 33 year old Brendan Eich built the first prototype of JavaScript in just ten days, originally codenamed Mocha (and then LiveScript). On December 4, 1995, Netscape and Sun Microsystems officially announced 'JavaScript' in a press release as "an easy-to-use object scripting language designed for creating live online applications that link together objects and resources on both clients and servers."
Over thirty years, JavaScript has cemented its place at the heart of the Web platform, and more broadly in desktop apps, operating systems (e.g. Windows' use of React Native), mobile apps, and even on microcontrollers.
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❤13👍4🤔1
CHALLENGE
const map = new Map([
['a', 1],
['b', 2],
['c', 3]
]);
const key = { id: 'key' };
map.set(key, 4);
map.set(key, 5);
const result = [];
result.push(map.get('a'));
result.push(map.get(key));
result.push(map.size);
result.push(map.has({ id: 'key' }));
console.log(result);
What is the output?
Anonymous Quiz
32%
[1, 5, 4, true]
28%
[1, 4, 4, false]
27%
[1, 5, 4, false]
13%
[1, 5, 5, true]
❤2👍2🔥2🤔1
CHALLENGE
const promise1 = Promise.resolve(10);
const promise2 = promise1.then(x => x * 2);
const promise3 = promise2.then(x => {
console.log(x);
return x + 5;
});
const promise4 = promise2.then(x => {
console.log(x);
return x * 3;
});
Promise.all([promise3, promise4]).then(results => {
console.log(results);
});
What is the output?
Anonymous Quiz
39%
20 20 [25, 60]
24%
10 10 [15, 30]
19%
10 20 [25, 60]
17%
20 25 [25, 60]
❤3👍3🤔2🤩2
CHALLENGE
const x = 5;
const y = 10;
const obj = {
x,
y,
z: x + y,
calculate() {
return this.x * this.y;
},
[x + y]: 'computed'
};
console.log(obj.calculate() + obj[15] + obj.z);
❤3
❤10
CHALLENGE
const curry = (fn) => {
return function curried(...args) {
if (args.length >= fn.length) {
return fn.apply(this, args);
}
return (...nextArgs) => curried(...args, ...nextArgs);
};
};
const multiply = (a, b, c) => a * b * c;
const curriedMultiply = curry(multiply);
const step1 = curriedMultiply(2);
const step2 = step1(3);
const result = step2(4);
console.log(result);👍3❤2
CHALLENGE
const a = { x: 1 };
const b = a;
const c = { x: 1 };
b.x = 2;
const d = b;
d.x = 3;
console.log(a.x);
console.log(b.x);
console.log(c.x);
console.log(a === b);
console.log(a === c);❤3
What is the output?
Anonymous Quiz
18%
2 2 1 false false
46%
3 3 1 true false
25%
1 2 1 true false
11%
3 3 3 true true
❤6👍4
⚠️ 🔵 Denial of Service and Source Code Exposure in React Server Components
Security researchers have found and disclosed two additional vulnerabilities in React Server Components while attempting to exploit the patches in last week’s critical vulnerability.
If you already updated for the Critical Security Vulnerability last week, you will need to update again.
If you updated to 19.0.2, 19.1.3, and 19.2.2, these are incomplete and you will need to update again.
December 11, 2025 by The React Team
Security researchers have found and disclosed two additional vulnerabilities in React Server Components while attempting to exploit the patches in last week’s critical vulnerability.
If you already updated for the Critical Security Vulnerability last week, you will need to update again.
If you updated to 19.0.2, 19.1.3, and 19.2.2, these are incomplete and you will need to update again.
December 11, 2025 by The React Team
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👍3🤔2
CHALLENGE
class Subject {
constructor() {
this.observers = [];
}
attach(observer) {
this.observers.push(observer);
}
notify(data) {
this.observers.forEach(obs => obs.update(data));
}
}
const subject = new Subject();
subject.attach({ update: (d) => console.log(d * 2) });
subject.attach({ update: (d) => console.log(d + 5) });
subject.notify(10);❤1
👍4❤2🤩1
Because making new frameworks like Next.js is relatively easy nowadays, someone with enough social influence can literally destroy Vercel's business by popularising some other framework, which, by the way, would have nearly identical problems anyway, because this was mostly the React problem itself.
Tigran Bayburtsyan
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CHALLENGE
const obj = { a: 1, b: 2, c: 3 };
Object.defineProperty(obj, 'd', {
value: 4,
enumerable: false
});
const entries = Object.entries(obj);
const keys = Object.keys(obj);
const values = Object.values(obj);
console.log(entries.length + keys.length + values.length);❤4🤩1