为什么Object.keys比hasOwnProperty更快？

Question

 // hasOwnProperty approach 
 for (key in obj) {
  if (obj.hasOwnProperty(key)) {
    value = obj[key];
  }
}

// Object.keys approach 
keys = Object.keys(obj);
for (i = 0, l = keys.length; i < l; i++) {
  value = obj[keys[i]];
}

According to jsperf the Object.keys approach is 50%+ faster http://jsperf.com/object-keys-vs-hasownproperty/45

Why is this?

Answer 1

• Object.keys looks up all own , enumerable properties (oh, and arrays are fast).
• for in additionally looks up inherited enumerable properties, not only own ones
• for in + hasOwnProperty additionally tests all looked up properties for whether they are own properties.

Even if there are no inherited enumerable properties, it is still more work to do than not.

Answer 2

Most of the tests in that thread show hasownproperty to be faster than Object.keys() . However, for those tests, the amount of properties to cycle through is small (<20 properties).

Wherever the number of object properties is large (no exact number, tests with >100 keys showed Object.keys() as a clear winner), Object.keys() beats it.

See https://jsperf.com/object-keys-vs-hasownproperty/55 . You can go to the bottom of the page to see all revisions to the testing.