[Vlasenko Fedor]

Без рекурсии в 8-10 раз быстрее в node

const ar = Array.from({length: 1_000_000}, (_, i) => i)
let b;

console.time('V1');
b = ar.filter(num => [1, 2, 5].includes(+String(num).charAt(0)));
console.timeEnd('V1');

b = [];
console.time('V2');
for (let i = 0; i < ar.length; i++) {
    const as = String(ar[i])[0];
    if (as === '1' || as === '2' || as === '5') b.push(ar[i])
}
console.timeEnd('V2');
console.time('V3');
b = ar.filter(num => /^[125]/.test(num));
console.timeEnd('V3');
b = [];
const getFirstNumber = num => {
    while (num > 9) num = num / 10 | 0;
    return num;
}
console.time('V4');
for (let i = 0; i < ar.length; i++) {
    let f = getFirstNumber(ar[i]);
    if (f === 1 || f === 2 || f === 5) b.push(ar[i])
}
console.timeEnd('V4');

и в браузерах уже V4 впереди планеты