>>742

(function(){
let ary = [];
ary.length = 10000000;
for (let i = 0; i < ary.length; ++i) {
ary[i] = false; // null or false
}
let count = 5, sum = 0;
for (let k = 0; k < count; ++k) {
let time0 = Date.now();
for (let i = 0; i < ary.length; ++i) {
ary[i] = (Math.random() > 0.5);
}
for (let i = 0; i < ary.length; ++i) {
ary[i] = (Math.random() > 0.5);
}
let time1 = Date.now();
sum += time1 - time0;
}
alert(sum * 1.0 / count);
})();

【結果】
null: 738.8
false: 594