[英]How to check if a javascript object has a certain property
Suppose I have a javascript object like this: 假设我有一个像这样的javascript对象:
window.config
config.UI = {
"opacity": {
"_type": "float",
"_tag": "input",
"_value": "1",
"_aka": "opacity",
"_isShow":"1"
}
How can I judge if the "opacity" object has a property named "_test"? 如何判断“不透明度”对象是否具有名为“_test”的属性? like 喜欢
var c=config.ui.opacity;
for(var i in c)
{
//c[i]=="_test"?
}
How do I find out if it's assigned as well? 我怎么知道它是否也被分配了?
There are at least three ways to do this; 至少有三种方法可以做到这一点; which one you use is largely up to you and sometimes even a matter of style, though there are some substantive differences: 您使用哪一个很大程度上取决于您,有时甚至是风格问题,尽管存在一些实质性差异:
if..in
You can use if..in
: 你可以使用if..in
:
if ("_test" in config.UI.opacity)
...because when used in a test (as opposed to the special for..in
loop), in
tests to see if the object or its prototype (or its prototype's prototype, etc.) has a property by that name. ...因为在测试中使用时(与特殊的for..in
循环相反), in
测试中查看对象或其原型(或其原型的原型等)是否具有该名称的属性。
hasOwnProperty
If you want to exclude properties from prototypes (it doesn't much matter in your example), you can use hasOwnProperty
, which is a function all objects inherit from Object.prototype
: 如果要从原型中排除属性(在您的示例中并不重要),可以使用hasOwnProperty
,这是一个函数,所有对象都从Object.prototype
继承:
if (config.UI.opacity.hasOwnProperty("_test"))
Finally, you can just retrieve the property (even if it doesn't exist) and the decide what to do with the result by looking at the result; 最后,您可以检索属性(即使它不存在),并通过查看结果来决定如何处理结果; if you ask an object for the value of a property it doesn't have, you'll get back undefined
: 如果你向一个对象询问它没有的属性的值,你将得到undefined
:
var c = config.UI.opacity._test;
if (c) {
// It's there and has a value other than undefined, "", 0, false, or null
}
or 要么
var c = config.UI.opacity._test;
if (typeof c !== "undefined") {
// It's there and has a value other than undefined
}
If it's possible that config.UI
won't have an opacity
property at all, you can make all of those more defensive: 如果config.UI
可能根本没有opacity
属性,那么你可以使所有这些更具防御性:
// The if..in version:
if (config.UI.opacity && "_test" in config.UI.opacity)
// The hasOwnProperty version
if (config.UI.opacity && config.UI.opacity.hasOwnProperty("_test"))
// The "just get it and then deal with the result" version:
var c = config.UI.opacity && config.UI.opacity._test;
if (c) { // Or if (typeof c !== "undefined") {
That last one works because the &&
operator is particularly powerful in JavaScript compared with some other languages; 最后一个是有效的,因为与其他语言相比, &&
运算符在JavaScript中特别强大; it's the corollary of the curiously-powerful ||
这是奇怪的强大||
的必然结果 operator . 运营商 。
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