Performance check about deleting property null assignment

bench.js

var now;
var obj, i;
var count = 10000000;
var key = 'key';

obj = {};
now = Date.now();
for (i = 0; i < count; i++) {
  obj[key] = 1;
  obj[key] = null;
}
console.log('null assignment(`obj[key] = null`):\n  %d ms', Date.now() - now);

obj = {};
now = Date.now();
for (i = 0; i < count; i++) {
  obj[key] = 1;
  delete obj[key];
}
console.log('deleting property(`delete obj[key]`):\n  %d ms', Date.now() - now);