const obj = {};
const map = new Map();
for (let i = 0; i<10000;i++) {
obj['a' + i] = i;
map.set('a' + i, i);
}
for (let i = 0; i<5000; i++) {
const j = i*2;
delete obj['a' + j];
obj['a' + j] = j;
}
for (let i = 0; i<5000; i++) {
const j = i*2;
map.delete('a' + j);
map.set('a' + j, j);
}