Also performance test:

const ITERATIONS_NUM = Math.pow(10, 8);

function testWithAllocation(i) {
    new Vector3(i + 1.13, i + 5.231, i + 7.1247);
}

function testWithoutAllocation(i) {
    if (!this.vec) {
        this.vec = new Vector3(0, 0, 0);
    }

    this.vec.set(i + 1.13, i + 5.231, i + 7.1247);
}

console.time('with-allocation');
for (let i = 0; i < ITERATIONS_NUM; ++i) {
    testWithAllocation(i);
}
console.timeEnd('with-allocation');

console.time('without-allocation');
let vec = new Vector3(0, 0, 0);
for (let i = 0; i < ITERATIONS_NUM; ++i) {
    testWithoutAllocation(i);
}
console.timeEnd('without-allocation');

with-allocation: 912.862ms
without-allocation: 65.783ms

And how it looks like with static syntax to avoid unwanted allocations:

function testWithoutAllocation(i) {
    static let vec = new Vector(0, 0, 0);
    vec.set(i + 1.13, i + 5.231, i + 7.1247);
}

I've added answers from our discussion to the repo: https://github.com/yehoryatskevych/proposal-function-static-variables/