Object descriptor getter/setter performance in recent Chrome/V8 versions

问题

Given

var obj = {};

var _a = 1;

obj._a = 1;

obj.aGetter = function() {
  return _a;
}

obj.aSetter = function(val) {
  _a = val;
}

Object.defineProperty(obj, 'a', {
  enumerable: true,
  get: function () {
    return _a;  
  },
  set: function(val) {
    _a = val;
  }     
});

using getter/setter functions

obj.aSetter(2);
obj.aGetter();

will have some decrease in Chrome/V8 performance (~3x) when compared to direct property access:

obj._a = 2;
obj._a;

This is be understandable. And using descriptor getter/setter

obj.a = 2;
obj.a;

will cause ~30x decrease in Chrome (41 to latest) performance - almost as slow as Proxy. While Firefox and older Chrome versions use descriptor getter/setter with no significant performance penalty.

What is the exact problem with descriptor getter/setter performance in recent Chrome/V8 versions? Is it a known issue that can be monitored?

The measurements were done with Benchmark.js (jsPerf engine). I'm unable to provide a link to jsPerf test to visualize the difference because jsPerf has been seriously screwed up with its anti-DDoS measures, but I'm sure there are existing ones that can prove a point.

回答1:

The changes in performance are relevant to this Chromium issue (credits go to @VyacheslavEgorov).

To avoid performance issues, a prototype should be used instead. This is one of few reasons why singleton classes may be used to instantiate an object once.

With ES5:

var _a = 1;

function Obj() {}

Object.defineProperty(Obj.prototype, 'a', {
  enumerable: true,
  get: function () {
    return _a;  
  },
  set: function(val) {
    _a = val;
  }     
});

var obj = new Obj();
// or
var obj = Object.create(Obj.prototype);

Or with ES6 syntactic sugar:

class Obj {
  constructor() {
    this._a = 1;
  }

  get a() {
    return this._a;
  }

  set a(val) {
    this._a = val;
  }     
}

let obj = new Obj();

来源：https://stackoverflow.com/questions/36338289/object-descriptor-getter-setter-performance-in-recent-chrome-v8-versions

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google-chrome