CHALLENGE
const person = { name: "Carlos", scores: [10, 20, 30] };
const clone = { ...person };
clone.name = "Diana";
clone.scores.push(40);
const snapshot = Object.assign({}, person);
snapshot.name = "Elena";
snapshot.scores.push(50);
console.log(person.name);
console.log(person.scores.length);
console.log(clone.name);
console.log(clone.scores === person.scores);
๐3๐ฅ3โค2
What is the output?
Anonymous Quiz
28%
Carlos 5 Diana true
28%
Carlos 4 Diana false
33%
Carlos 3 Diana false
12%
Diana 5 Diana true
โค2๐2๐ค1
CHALLENGE
"use strict";
function createCounter() {
let count = 0;
return {
increment() { count++; },
decrement() { count--; },
getCount() { return count; },
reset: () => { count = 0; }
};
}
const counter = createCounter();
counter.increment();
counter.increment();
counter.increment();
counter.decrement();
const { getCount, reset } = counter;
console.log(getCount());
reset();
console.log(counter.getCount());
๐ฅ11โค3๐1
What is the output?
Anonymous Quiz
40%
2 0
26%
2 undefined
29%
TypeError: getCount is not a function
4%
3 3
CHALLENGE
const p1 = new Promise((resolve) => {
console.log("A");
resolve("X");
});
const p2 = p1.then((val) => {
console.log("B");
return val + "Y";
});
const p3 = p2.then((val) => {
console.log("C:", val);
});
console.log("D");
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โค6๐5๐ฅ1๐ค1
CHALLENGE
class AppError extends Error {
constructor(message, statusCode) {
super(message);
this.name = this.constructor.name;
this.statusCode = statusCode;
}
}
class ValidationError extends AppError {
constructor(message) {
super(message, 400);
this.fields = [];
}
}
function riskyOperation(value) {
if (value === null) throw new ValidationError("Null value");
if (value < 0) throw new AppError("Negative value", 422);
return value * 2;
}
const results = [];
for (const val of [10, null, -5, 3]) {
try {
results.push(riskyOperation(val));
} catch (e) {
if (e instanceof ValidationError) {
results.push(`Validation:${e.statusCode}`);
} else if (e instanceof AppError) {
results.push(`App:${e.statusCode}`);
} else {
results.push("Unknown");
}
}
}
console.log(results.join(" | "));
๐5๐ฅ2๐คฉ2
What is the output?
Anonymous Quiz
16%
20 | Unknown | App:422 | 6
59%
20 | Validation:400 | App:422 | 6
17%
20 | App:400 | App:422 | 6
8%
20 | Validation:400 | Validation:422 | 6
โค4๐ฅ2
JavaScriptโs date/time handling is notoriously messy and libraries like Moment.js became popular as a way to work around it. In 2017, Maggie Johnson-Pint, a maintainer of Moment.js, proposed the Temporal API to fix date/time handling for good, and weโre mostly there (support is growing, with Safari and Node to catch up).
Jason Williams (Bloomberg)
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โค6๐3
CHALLENGE
const p1 = new Promise((resolve) => {
console.log("A");
resolve("B");
});
const p2 = p1.then((val) => {
console.log(val);
return "C";
});
p2.then((val) => {
console.log(val);
});
console.log("D");
๐5๐ฅ3โค2
โค3๐1๐ฅ1
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๐คฃ5๐ฅ4โค2๐2๐ค1
CHALLENGE
function Vehicle(make, model, year) {
this.make = make;
this.model = model;
this.year = year;
this.describe = function () {
return `${this.year} ${this.make} ${this.model}`;
};
}
Vehicle.prototype.age = function (currentYear) {
return currentYear - this.year;
};
const car = new Vehicle("Toyota", "Supra", 1998);
const bike = new Vehicle("Harley", "Sportster", 2005);
console.log(car.describe());
console.log(bike.age(2025));
console.log(car.constructor === Vehicle);
console.log(Object.getPrototypeOf(car) === Vehicle.prototype);
๐4๐ฅ3
What is the output?
Anonymous Quiz
17%
1998 Toyota Supra 27 false true
51%
1998 Toyota Supra 20 true true
19%
1998 Toyota Supra 20 true false
14%
Toyota Supra 1998 27 true true
โค4๐2๐ฅ2
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โค9๐ฅ3๐2๐ค2
CHALLENGE
class EventEmitter {
#listeners = new WeakMap();
#registry = new FinalizationRegistry((label) => {
console.log(`Cleaned up: ${label}`);
});
subscribe(target, callback) {
if (!this.#listeners.has(target)) {
this.#listeners.set(target, []);
}
this.#listeners.get(target).push(callback);
this.#registry.register(target, target.name ?? "unknown");
}
emit(target) {
const cbs = this.#listeners.get(target);
if (cbs) cbs.forEach(cb => cb());
}
}
const emitter = new EventEmitter();
let obj1 = { name: "sensor" };
let obj2 = { name: "timer" };
const ws = new WeakSet([obj1, obj2]);
emitter.subscribe(obj1, () => console.log("sensor fired"));
emitter.subscribe(obj2, () => console.log("timer fired"));
emitter.subscribe(obj1, () => console.log("sensor logged"));
emitter.emit(obj1);
console.log(ws.has(obj1));
obj1 = null;
console.log(ws.has({ name: "sensor" }));โค3๐2๐ฅ2
What is the output?
Anonymous Quiz
24%
sensor fired sensor logged true false
32%
sensor fired sensor logged false false
35%
sensor fired timer fired sensor logged true false
9%
sensor fired sensor logged true true
โค5๐1
The Node.js team has long been discussing shifting Node to a new schedule of one major release per year (instead of two), removing the odd/even distinction, and making every release LTS (with a prior 11 months of alpha/current status). This is a preview post not intended for final publication till April, so things are subject to change (backup version).
The Node.js Team
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โค5๐2๐ฅ2
CHALLENGE
const temperature = {
celsius: 22,
[Symbol.toPrimitive](hint) {
if (hint === 'number') {
return this.celsius;
}
if (hint === 'string') {
return `${this.celsius}ยฐC`;
}
return this.celsius + 273.15;
}
};
console.log(`Temp: ${temperature}`);
console.log(temperature + 0);
console.log(temperature * 2);
console.log(+temperature);
โค5๐ฅ5๐2