[Javascript] Performance: Add large number of array elements to set.

Test environment: Node 12.18.2 on Intel(R) Xeon(R) CPU E5-2697 v3 @ 2.60GHz

const SZ = 99999999


const l2a = [];
for (let i = 0; i < 10000; i++) {
    const sub = [];
    for (let j = 0; j < 10000; j++) {
        sub.push(j);
    }
    l2a.push(sub);
}

let s = new Set();
let t = -Date.now();
for (const a of l2a) {
    for (const n of a) {
        s.add(n);
    }
}
t += Date.now();
console.log('Takes: ', t);

const ta = [];
t = -Date.now();
for (const a of l2a) {
    ta.concat(...a);
}
s = new Set(ta);
t += Date.now();
console.log('Takes: ', t);

Result is

Takes:  3070
Takes:  1665

Concating and expanding array are very fast comparing to calling a function(Set.add).