我一直假设对象的键存储为字符串,并且将转换任何非字符串值。因此,正是在这种假设下,在编写一些必须为数千个键存储一个小值的代码时,我将所有键都转换为基数 36:
// theKey is an integer
myMap[theKey.toString(36)] = theValue;
然后,我决定看看我的假设是否真的正确,并使用 Chrome 的分析器检查内存使用情况。大致如下是我运行的测试和内存使用情况:
window.objIntegers = {};
for (i = 100000; i--) window.objIntegers[i] = 'a';
// 786kb
window.objStrings = {};
for (i = 100000; i--) window.objStrings[i.toString(36)] = 'a';
// 16.7mb!
// and the same pattern but with:
key = i + .5; // 16.7mb
key = i + ''; // 786kb
key = '0' + i; // 16.7mb
key = i + '0'; // 16.7mb
显然,我的假设是错误的。不过,我想知道的是,它们是如何存储的,这种行为是标准的,还是只是 Chromium/WebKit 团队添加的一些额外的技巧?
最佳答案
这确实是V8的一些额外技巧。
JSObject
(JS Object
的内部 C++ 表示)有两个属性,elements
和 properties
,其中“元素”是带有数字索引的 JS 属性,而“属性”是带有字符串索引的 JS 属性。
显然,数字索引在这里消耗的内存要少得多,因为不需要存储属性名称。
http://code.google.com/intl/de-DE/chrome/devtools/docs/memory-analysis-101.html#primitive_objects
A typical JavaScript object posesses two arrays: one for storing named properties, another for storing numeric elements.
这个从v8源码可以看出:
http://code.google.com/p/v8/source/browse/trunk/src/objects.h#1483
// [properties]: Backing storage for properties.
...
// [elements]: The elements (properties with names that are integers).
http://code.google.com/p/v8/source/browse/trunk/src/runtime.cc#4462
MaybeObject* Runtime::SetObjectProperty(Isolate* isolate,
Handle<Object> object,
Handle<Object> key,
Handle<Object> value,
PropertyAttributes attr,
StrictModeFlag strict_mode) {
...
// Check if the given key is an array index.
uint32_t index;
if (key->ToArrayIndex(&index)) {
// In Firefox/SpiderMonkey, Safari and Opera you can access the characters
// of a string using [] notation. We need to support this too in
// JavaScript.
// In the case of a String object we just need to redirect the assignment to
// the underlying string if the index is in range. Since the underlying
// string does nothing with the assignment then we can ignore such
// assignments.
if (js_object->IsStringObjectWithCharacterAt(index)) {
return *value;
}
Handle<Object> result = JSObject::SetElement(
js_object, index, value, attr, strict_mode, set_mode);
if (result.is_null()) return Failure::Exception();
return *value;
}
if (key->IsString()) {
Handle<Object> result;
if (Handle<String>::cast(key)->AsArrayIndex(&index)) {
result = JSObject::SetElement(
js_object, index, value, attr, strict_mode, set_mode);
} else {
Handle<String> key_string = Handle<String>::cast(key);
key_string->TryFlatten();
result = JSReceiver::SetProperty(
js_object, key_string, value, attr, strict_mode);
}
if (result.is_null()) return Failure::Exception();
return *value;
}
// Call-back into JavaScript to convert the key to a string.
bool has_pending_exception = false;
Handle<Object> converted = Execution::ToString(key, &has_pending_exception);
if (has_pending_exception) return Failure::Exception();
Handle<String> name = Handle<String>::cast(converted);
if (name->AsArrayIndex(&index)) {
return js_object->SetElement(
index, *value, attr, strict_mode, true, set_mode);
} else {
return js_object->SetProperty(*name, *value, attr, strict_mode);
}
}
我不会详细介绍,但请注意 SetObjectProperty
调用 SetElement
或 SetProperty
,具体取决于 key 。不确定为什么在您的测试用例 key = i + '0'
中检查失败。
关于javascript - 对象的属性名称如何存储在 Javascript 中?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/9976498/