console.time('Fibonacci');
const n = 1000;
let a = 0n, b = 1n;

if (n === 1) {
    a = 0n;
} else if (n === 2) {
    b = 1n;
} else {
    for (let i = 3; i <= n; i++) {
        [a, b] = [b, a + b];
    }
}

console.timeEnd('Fibonacci');
console.log(`Последнее число: ${n === 1 ? a.toString() : b.toString()}`);