Run results for: conditional self-assignment vs. membership check with in vs. membership check with hasOwnProperty vs. null check

Run details:

User agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/114.0.0.0 Safari/537.36

Browser: Chrome 114

Operating system: Mac OS X 10.15.7

Device Platform: Desktop

Date tested: one year ago

Test name	Executions per second
hasOwnProperty membership check	1957.8 Ops/sec
self-assignment	2802.4 Ops/sec
in membership check	6026.8 Ops/sec
null check	5747.1 Ops/sec

Script Preparation code:

var x = {
    foo: []
};
var keys = Array.from({
    length: 5000
}).map((x, i) => i < 2500 ? i : i - 2500);

Tests:

hasOwnProperty membership check
for (const key of keys) { if (!Object.prototype.hasOwnProperty.call(x, key)) { x[key] = []; } }
self-assignment
for (const key of keys) { x[key] = x[key] ?? []; }
in membership check
for (const key of keys) { if (!(key in x)) { x[key] = []; } }
null check
for (const key of keys) { if (x[key] == null) { x[key] = []; } }