const dest = (new Array(1000)).fill(0);
const source = (new Array(1000)).fill(1);
for (const elm of source) {
dest.push(elm);
}
const dest = (new Array(1000)).fill(0);
const source = (new Array(1000)).fill(1);
dest.push.apply(dest, source);
const dest = (new Array(1000)).fill(0);
const source = (new Array(1000)).fill(1);
dest.push(...source);
let dest = (new Array(1000)).fill(0);
const source = (new Array(1000)).fill(1);
dest = dest.concat(source);