JavaScript test
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Javascript и все что с ним связано

Ссылка: @Portal_v_IT

Сотрудничество: @oleginc
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❗️Что будет на выходе:

const clothes = [ 'jacket', 't-shirt' ];
clothes.length = 0;
console.log(clothes[0]);

Ответ: undefined

JavaScript test | #JavaScript
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❗️Что будет на выходе:

const weakMap = new WeakMap();
const obj1 = {};
const obj2 = { key: 'value' };

weakMap.set(obj1, obj2);

const result = weakMap.get(obj1).key.split('').reverse().join('');

console.log(result);

Ответ: "eulav"

JavaScript test | #JavaScript
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❗️Что будет на выходе:

for (var i =0; i < 10; i++){
setTimeout(function (){
console.log(i);
}, 0);
}

Ответ: 10 10 10 10 10 10 10 10 10 10

JavaScript test | #JavaScript
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❗️Что будет на выходе:

const x = 0.1 + 0.2;
const y = 0.3;

console.log(x === y);
console.log(x.toFixed(1) === y.toFixed(1));
console.log(+x.toFixed(1) === +y.toFixed(1));

const num = 42;
console.log(num.toString(2));
console.log(parseInt('101010', 2));

Ответ: false true true 101010 42

JavaScript test | #JavaScript
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❗️Что будет на выходе:

const x = 5;
const y = 10;

const result = `${x + y}`;
const nested = `Value: ${`${x}` + `${y}`}`;
const expr = `${x}${y}`;

console.log(result);
console.log(nested);
console.log(expr);
console.log(typeof result);

Ответ: 15 Value: 510 510 string

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❗️Что будет на выходе:

const obj = Object.seal({ a: 1, b: { c: 2 } });
obj.a = 10;
obj.b.c = 20;
obj.d = 30;
delete obj.a;

const frozen = Object.freeze({ x: 5, y: { z: 10 } });
frozen.x = 50;
frozen.y.z = 100;
delete frozen.y;

console.log(obj.a, obj.b.c, obj.d, frozen.x, frozen.y.z);

Ответ: 10 20 undefined 5 100

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❗️Что будет на выходе:

console.log('1');

setTimeout(() => console.log('2'), 0);

Promise.resolve().then(() => console.log('3'));

queueMicrotask(() => console.log('4'));

setTimeout(() => console.log('5'), 0);

Promise.resolve().then(() => {
console.log('6');
return Promise.resolve();
}).then(() => console.log('7'));

console.log('8');

Ответ: 1 8 3 4 6 7 2 5

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❗️Что будет на выходе:

const array = Array.from({ length: 5 }, () => Math.random() > 0.5);

console.log(array);

Ответ: An array of 5 random true/false values

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❗️Что будет на выходе:

const obj = {
name: 'Sarah',
getName() { return this.name; },
getNameArrow: () => this.name
};

const { getName, getNameArrow } = obj;
const boundGetName = obj.getName.bind(obj);

console.log(getName());
console.log(getNameArrow());
console.log(boundGetName());
console.log(obj.getName());
console.log(obj.getNameArrow());

Ответ: undefined undefined Sarah Sarah undefined

JavaScript test | #JavaScript
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❗️Что будет на выходе:

const array = Array.from({ length: 5 }, () => Math.random() > 0.5);

console.log(array);

Ответ: An array of 5 random true/false values

JavaScript test | #JavaScript
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❗️Что будет на выходе:

const memoryLeak = () => {
const cache = new Map();
const weakCache = new WeakMap();

const objKey = { id: 123 };
const data = { name: 'User data', value: 42 };

cache.set(objKey, data);
weakCache.set(objKey, data);

// Simulate removing reference to key
const result = { map: cache.has(objKey), weakMap: weakCache.has(objKey) };
// objKey = null; // This would be an error, as const can't be reassigned

return result;
};

console.log(memoryLeak());

Ответ: { map: true, weakMap: true }

JavaScript test | #JavaScript
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❗️Что будет на выходе:

async function foo() {
console.log('Start');
await Promise.resolve().then(() => {
console.log('Inside Promise');
});
console.log('End');
}

foo();
console.log('Outside');

Ответ: Start, Outside, Inside, Promise, End

JavaScript test | #JavaScript
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❗️Что будет на выходе:

const symbol1 = Symbol('desc1');
const symbol2 = Symbol('desc2');
const myObject = {};
myObject[symbol1] = 'Value1';
myObject[symbol2] = 'Value2';
let output = '';
for (let key in myObject) {
output += myObject[key] + ' ';
}
output += Object.getOwnPropertySymbols(myObject).length;
console.log(output);

Ответ: 2

JavaScript test | #JavaScript
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❗️Что будет на выходе:

const getValue = (x) => {
console.log(`Getting: ${x}`);
return x;
};

const obj = { name: null };

const result = obj.name || getValue('default') && getValue('final');
console.log(`Result: ${result}`);

Ответ: Getting: default Getting: final Result: final

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❗️Что будет на выходе:

const array = [1, 2, 3, 4, 5];
const result = array.filter(n => n % 2).map(n => n * 2);

console.log(result);

Ответ: [2, 6, 10]

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❗️Что будет на выходе:

function createCounter() {
let count = 0;
return function(increment = 1) {
count += increment;
return count;
};
}

const counter1 = createCounter();
const counter2 = createCounter();

console.log(counter1());
console.log(counter1(5));
console.log(counter2(3));
console.log(counter1());
console.log(counter2());

Ответ: 1 6 3 7 4

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❗️Что будет на выходе:

Promise.resolve(1)
.then(r => Promise.reject(r))
.catch(console.log)
.then(() => console.log(2));

Ответ: 1, 2

JavaScript test | #JavaScript
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❗️Что будет на выходе:

let obj1 = { name: 'Sarah' };
let obj2 = { name: 'Mike' };
let obj3 = { ref: obj1 };

obj1.circular = obj1;
obj2.partner = obj3;
obj3.partner = obj2;

let weakMap = new WeakMap();
weakMap.set(obj1, 'data1');
weakMap.set(obj2, 'data2');

obj1 = null;
obj2 = null;

console.log(weakMap.has(obj3.ref));
console.log(obj3.partner.name);

Ответ: true Mike

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❗️Что будет на выходе:

const arr = [1, 2, 3, 4, 5];
const result = arr
.map(x => x * 2)
.filter(x => x > 5)
.reduce((acc, val) => {
return acc + (val % 3 === 0 ? val : 0);
}, 0);

console.log(result);

Ответ: 6

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