Has a bit of a 8-bit Game Boy Color vibe to it. You can create games, and try some examples, in this online playground.
Charles Cailleteau
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β€6π€5π1π₯1π€©1
CHALLENGE
function process(data) {
try {
if (!data) {
throw new TypeError('No data provided');
}
if (Array.isArray(data)) {
return data.map(item => item * 2);
}
if (typeof data === 'object') {
return Object.keys(data);
}
return data.toString();
} catch (error) {
if (error instanceof TypeError) {
return 'Type error occurred';
}
return 'Unknown error';
}
}
console.log(process(null));
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What is the output?
Anonymous Quiz
26%
null
32%
TypeError: Cannot read properties of null (reading 'toString')
35%
Type error occurred
8%
Unknown error
β€2π€£1
CHALLENGE
function Animal(name) {
this.name = name;
}
Animal.prototype.speak = function() {
return `${this.name} makes a noise.`;
};
function Dog(name) {
Animal.call(this, name);
}
Dog.prototype = Object.create(Animal.prototype);
Dog.prototype.constructor = Dog;
Dog.prototype.speak = function() {
return `${this.name} barks.`;
};
const animal = new Animal('Rover');
const dog = new Dog('Rex');
console.log(dog instanceof Animal, dog.speak(), animal.speak(), Dog.prototype.isPrototypeOf(dog));
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What is the output?
Anonymous Quiz
35%
true 'Rex barks.' 'Rover makes a noise.' true
34%
false 'Rex barks.' 'Rover makes a noise.' true
22%
true 'Rex makes a noise.' 'Rover makes a noise.' true
9%
true 'Rex barks.' 'Rover makes a noise.' false
π5β€3π€©3π₯2
CHALLENGE
function highlight(strings, ...values) {
return strings.reduce((result, str, i) => {
const value = values[i] ? `<span>${values[i]}</span>` : '';
return result + str + value;
}, '');
}
const language = 'JavaScript';
const years = 10;
const result = highlight`I have been coding in ${language} for ${years} years`;
console.log(result);
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What is the output?
Anonymous Quiz
20%
I have been coding in <span>JavaScript</span> for <span>10</span>
39%
I have been coding in <span>JavaScript</span> for <span>10</span> years
31%
I have been coding in JavaScript for 10 years
10%
I have been coding in <span>JavaScript</span> for <span>10</span> yearsundefined
β€3π3
CHALLENGE
function createCounter() {
let count = 0;
return {
increment() {
count++;
return count;
},
decrement() {
count--;
return count;
},
getValue() {
return count;
}
};
}
const counter1 = createCounter();
const counter2 = createCounter();
counter1.increment();
counter1.increment();
counter2.increment();
counter1.decrement();
console.log(counter1.getValue() + counter2.getValue());
β€5π5π€£3π₯1
π9π€1
π Exploring JavaScript (ES2025 Edition)
Dr. Axel is back with his latest book covering all things relating to modern JavaScript at the language level (think built-in data types, modularity, how objects, classes and promises work, etc.). As with all of Axel's books, itβs available to buy but also to read online in HTML form for free. Heβs also produced a set of flashcards to help you learn language features in both HTML and Anki forms.
Dr. Axel Rauschmayer
Dr. Axel is back with his latest book covering all things relating to modern JavaScript at the language level (think built-in data types, modularity, how objects, classes and promises work, etc.). As with all of Axel's books, itβs available to buy but also to read online in HTML form for free. Heβs also produced a set of flashcards to help you learn language features in both HTML and Anki forms.
Dr. Axel Rauschmayer
π₯4β€3π2
CHALLENGE
class ShoppingCart {
constructor() {
if (ShoppingCart.instance) {
return ShoppingCart.instance;
}
this.items = [];
ShoppingCart.instance = this;
}
addItem(item) {
this.items.push(item);
}
getItems() {
return [...this.items];
}
}
const cart1 = new ShoppingCart();
const cart2 = new ShoppingCart();
cart1.addItem('Book');
cart2.addItem('Laptop');
console.log(cart1.getItems());
What is the output?
Anonymous Quiz
11%
['Laptop']
25%
TypeError: Cannot read property 'push' of undefined
43%
['Book', 'Laptop']
21%
['Book']
π6π€2β€1π₯1
The creator of the Porffor JavaScript compiler talks about the various ways to make JavaScript faster to execute, before digging into Porfforβs approach.
Oliver Medhurst
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β€5π3π₯1
CHALLENGE
const templateFn = (strings, ...values) => {
return strings.reduce((result, str, i) => {
const value = values[i] !== undefined ?
(typeof values[i] === 'number' ? values[i] * 2 : values[i]) : '';
return result + str + value;
}, '');
};
const num = 5;
const str = 'world';
const result = templateFn`Hello ${str}, ${num} times ${'!'}`;
console.log(result);
π5β€2
What is the output?
Anonymous Quiz
20%
Hello world, 10 times !undefined
28%
Hello world, 5 times !undefined
36%
Hello world, 10 times !
15%
Hello world, 5 times !
π€6β€5π4
A custom element that uses the CSS Custom Highlight API (supported by most modern browsers) for syntax highlighting so you donβt need to retreat to the age-old method of wrapping every token in spans.
AndrΓ© Ruffert
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π5β€3π₯1
CHALLENGE
function processConfig(config) {
const settings = {
timeout: config.timeout ?? 1000,
retries: config.retries ?? 3,
logging: config.logging ?? false,
debug: config.debug || true
};
return settings;
}
const userConfig = {
timeout: 0,
retries: null,
logging: false,
debug: false
};
console.log(processConfig(userConfig));
π8β€1
What is the output?
Anonymous Quiz
21%
{ timeout: 1000, retries: null, logging: false, debug: true }
29%
{ timeout: 1000, retries: 3, logging: false, debug: false }
39%
{ timeout: 0, retries: 3, logging: false, debug: true }
11%
{ timeout: 0, retries: 3, logging: false, debug: false }
π₯7β€3π3
CHALLENGE
const createMathOps = (base) => {
return {
add: (x) => base + x,
multiply: (x) => base * x
};
};
const createAdvancedMathOps = (base) => {
const basicOps = createMathOps(base);
return {
...basicOps,
square: () => basicOps.multiply(base),
addThenSquare: (x) => {
const added = basicOps.add(x);
return added * added;
}
};
};
const calculator = createAdvancedMathOps(5);
console.log(calculator.addThenSquare(3));
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