鉴于这种非常熟悉的原型构造模型:
function Rectangle(w,h) {
this.width = w;
this.height = h;
}
Rectangle.prototype.area = function() {
return this.width * this.height;
};
有人可以解释为什么在没有'new'关键字的情况下调用new Rectangle(2,3)
比调用Rectangle(2,3)
快10倍?我会假设因为new会因为涉及到原型而增加了函数执行的复杂性,所以它会更慢。
示例:
var myTime;
function startTrack() {
myTime = new Date();
}
function stopTrack(str) {
var diff = new Date().getTime() - myTime.getTime();
println(str + ' time in ms: ' + diff);
}
function trackFunction(desc, func, times) {
var i;
if (!times) times = 1;
startTrack();
for (i=0; i<times; i++) {
func();
}
stopTrack('(' + times + ' times) ' + desc);
}
var TIMES = 1000000;
trackFunction('new rect classic', function() {
new Rectangle(2,3);
}, TIMES);
trackFunction('rect classic (without new)', function() {
Rectangle(2,3);
}, TIMES);
收益率(在Chrome中):
(1000000 times) new rect classic time in ms: 33 (1000000 times) rect classic (without new) time in ms: 368 (1000000 times) new rect classic time in ms: 35 (1000000 times) rect classic (without new) time in ms: 374 (1000000 times) new rect classic time in ms: 31 (1000000 times) rect classic (without new) time in ms: 368
答案 0 :(得分:17)
当您调用没有“new”的函数时,您怀疑“this”指向的是什么?它将是“窗口”。更新比使用“new”调用它时更新新构建的新对象要慢。
将第二个版本更改为:
trackFunction('rect classic (without new)', function() {
Rectangle.call({}, 2,3);
}, TIMES);
看看你得到了什么。另一件事是:
trackFunction('rect with constant object', (function() {
var object = { height: 0, width: 0 };
return function() {
Rectangle.call(object, 2, 3);
};
})());
这将节省每次迭代重建虚拟对象的成本。