javascript中的全局对象

Question

如果我在 chrome 中运行此代码：

for(var i in window){console.log(i)}

我得到了很多属性，如下所示。

但是如果我运行这段代码：

console.log(window.hasOwnProperty("parseInt"))
console.log(window.hasOwnProperty("undefined"))

我都得到了true。

.hasOwnProperty() 怎么可能返回 true，但它没有出现在 for-in 循环生成的属性列表中？

循环结果：

top VM1531:2
window VM1531:2
location VM1531:2
external VM1531:2
chrome VM1531:2
document VM1531:2
getInnerFunction VM1531:2
theInnerFunction VM1531:2
__commandLineAPI VM1531:2
i VM1531:2
speechSynthesis VM1531:2
localStorage VM1531:2
sessionStorage VM1531:2
applicationCache VM1531:2
webkitStorageInfo VM1531:2
indexedDB VM1531:2
webkitIndexedDB VM1531:2
crypto VM1531:2
CSS VM1531:2
performance VM1531:2
console VM1531:2
devicePixelRatio VM1531:2
styleMedia VM1531:2
parent VM1531:2
opener VM1531:2
frames VM1531:2
self VM1531:2
defaultstatus VM1531:2
defaultStatus VM1531:2
status VM1531:2
name VM1531:2
length VM1531:2
closed VM1531:2
pageYOffset VM1531:2
pageXOffset VM1531:2
scrollY VM1531:2
scrollX VM1531:2
screenTop VM1531:2
screenLeft VM1531:2
screenY VM1531:2
screenX VM1531:2
innerWidth VM1531:2
innerHeight VM1531:2
outerWidth VM1531:2
outerHeight VM1531:2
offscreenBuffering VM1531:2
frameElement VM1531:2
clientInformation VM1531:2
navigator VM1531:2
toolbar VM1531:2
statusbar VM1531:2
scrollbars VM1531:2
personalbar VM1531:2
menubar VM1531:2
locationbar VM1531:2
history VM1531:2
screen VM1531:2
postMessage VM1531:2
close VM1531:2
blur VM1531:2
focus VM1531:2
ondeviceorientation VM1531:2
ondevicemotion VM1531:2
onautocompleteerror VM1531:2
onautocomplete VM1531:2
onunload VM1531:2
onstorage VM1531:2
onpopstate VM1531:2
onpageshow VM1531:2
onpagehide VM1531:2
ononline VM1531:2
onoffline VM1531:2
onmessage VM1531:2
onhashchange VM1531:2
onbeforeunload VM1531:2
onwaiting VM1531:2
onvolumechange VM1531:2
ontoggle VM1531:2
ontimeupdate VM1531:2
onsuspend VM1531:2
onsubmit VM1531:2
onstalled VM1531:2
onshow VM1531:2
onselect VM1531:2
onseeking VM1531:2
onseeked VM1531:2
onscroll VM1531:2
onresize VM1531:2
onreset VM1531:2
onratechange VM1531:2
onprogress VM1531:2
onplaying VM1531:2
onplay VM1531:2
onpause VM1531:2
onmousewheel VM1531:2
onmouseup VM1531:2
onmouseover VM1531:2
onmouseout VM1531:2
onmousemove VM1531:2
onmouseleave VM1531:2
onmouseenter VM1531:2
onmousedown VM1531:2
onloadstart VM1531:2
onloadedmetadata VM1531:2
onloadeddata VM1531:2
onload VM1531:2
onkeyup VM1531:2
onkeypress VM1531:2
onkeydown VM1531:2
oninvalid VM1531:2
oninput VM1531:2
onfocus VM1531:2
onerror VM1531:2
onended VM1531:2
onemptied VM1531:2
ondurationchange VM1531:2
ondrop VM1531:2
ondragstart VM1531:2
ondragover VM1531:2
ondragleave VM1531:2
ondragenter VM1531:2
ondragend VM1531:2
ondrag VM1531:2
ondblclick VM1531:2
oncuechange VM1531:2
oncontextmenu VM1531:2
onclose VM1531:2
onclick VM1531:2
onchange VM1531:2
oncanplaythrough VM1531:2
oncanplay VM1531:2
oncancel VM1531:2
onblur VM1531:2
onabort VM1531:2
onwheel VM1531:2
onwebkittransitionend VM1531:2
onwebkitanimationstart VM1531:2
onwebkitanimationiteration VM1531:2
onwebkitanimationend VM1531:2
ontransitionend VM1531:2
onsearch VM1531:2
getSelection VM1531:2
print VM1531:2
stop VM1531:2
open VM1531:2
alert VM1531:2
confirm VM1531:2
prompt VM1531:2
find VM1531:2
scrollBy VM1531:2
scrollTo VM1531:2
scroll VM1531:2
moveBy VM1531:2
moveTo VM1531:2
resizeBy VM1531:2
resizeTo VM1531:2
matchMedia VM1531:2
getComputedStyle VM1531:2
getMatchedCSSRules VM1531:2
webkitConvertPointFromPageToNode VM1531:2
webkitConvertPointFromNodeToPage VM1531:2
requestAnimationFrame VM1531:2
cancelAnimationFrame VM1531:2
webkitRequestAnimationFrame VM1531:2
webkitCancelAnimationFrame VM1531:2
webkitCancelRequestAnimationFrame VM1531:2
captureEvents VM1531:2
releaseEvents VM1531:2
btoa VM1531:2
atob VM1531:2
setTimeout VM1531:2
clearTimeout VM1531:2
setInterval VM1531:2
clearInterval VM1531:2
TEMPORARY VM1531:2
PERSISTENT VM1531:2
showModalDialog VM1531:2
webkitRequestFileSystem VM1531:2
webkitResolveLocalFileSystemURL VM1531:2
openDatabase VM1531:2
addEventListener VM1531:2
removeEventListener VM1531:2
dispatchEvent

Answer 1

可以通过将对象属性标记为“不可枚举”来从迭代机制中隐藏它们：

var foo = { a: "Hello", b: "World" };
Object.defineProperty(foo, "c", { value: "where am I?" });

默认情况下，使用defineProperty() 定义的属性是不可可枚举的，因此如果您在对象“foo”上运行for ... in 循环，您将看不到“c”。

当然，全局对象是具有各种神奇属性的东西，但基本的“可枚举”标志仍然适用。至于为什么究竟“parseInt”和“undefined”是不可枚举的，我不知道。

edit — cookie Monster 在评论中指出Object.getOwnPropertyNames() 函数将 返回不可枚举属性的名称。但是Object.keys() 不会。因此，您可以使用该差异来查看对象是否具有不可枚举的属性：

 function hasSneakyProperties(o) {
     return Object.getOwnPropertyNames(o).length !== Object.keys(o).length;
 }

我不知道为什么有人会这样做，但你去吧。