检查 JavaScript 对象是否为 null subtelty
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/18421995/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
Checking JavaScript object for null subtelty
提问by Peter Kellner
I believe I have found cases where I need to check for both undefined and null for a javascript object as follows:
我相信我已经找到了需要检查 javascript 对象的 undefined 和 null 的情况，如下所示：
if (x !== undefined && x != null && x.length > 0) {...}
However, in a recent upgrade of JetBrains tools, it tells me that this is sufficient
但是，在最近的 JetBrains 工具升级中，它告诉我这已经足够了
if (x != undefined && x.length > 0) {...}
My question is, I'm just wanting to insure that a string "x" has a length of non-zero and is not undefined or null (with the least amount tests).
我的问题是，我只是想确保字符串“x”的长度不为零，并且不是未定义的或空的（使用最少的测试）。
Thoughts?
想法？
回答by gdoron is supporting Monica
in javascript
在 JavaScript 中
undefined == null // true
undefined === null // false
so checking with ==for undefinedmakes the ==check for nullredundant.
所以检查==forundefined会使==检查变得null多余。
回答by Dennis
Try
尝试
if (x && x.length)
as undefined, nulland 0are all falsy values.
as undefined，null和0都是假值。
EDIT:
as you seem to know that xshould be a string, you can also just use if (x)as an empty string is also falsy.
编辑：正如你似乎知道x应该是 a string，你也可以只使用if (x)一个空字符串也是假的。
回答by Alaeddine
You can use _.isNullfrom Underscorea JavaScript library provides a whole mess of useful functional programming helpers.
你可以_.isNull从Underscore使用一个 JavaScript 库，它提供了一大堆有用的函数式编程助手。
_.isNull(object)
_.isNull(对象)
Returns true if the value of object is null.
如果 object 的值为 null，则返回 true。
_.isNull(null);
=> true
_.isNull(undefined);
=> false
回答by James Drinkard
This is what I use and it's the most concise. It covers: undefined, null, NaN, 0, "" (empty string), or false. We can therefore say that "object" is truthy.
这是我使用的，它是最简洁的。它涵盖：未定义、空值、NaN、0、""（空字符串）或 false。因此，我们可以说“对象”是真实的。
if(object){
doSomething();
}
回答by Wex
Checking if foo === undefinedwill trigger the error foo is not defined. See variable === undefined vs. typeof variable === "undefined"
检查是否foo === undefined会触发错误foo is not defined。请参阅变量 === 未定义与 typeof 变量 === “未定义”
The existential operatorin CoffeeScript compiles to
CoffeeScript 中的存在运算符编译为
typeof face !== "undefined" && face !== null
Edit:
编辑：
Matt's commentis better if you only want to check for strings:
如果您只想检查字符串，Matt 的评论会更好：
typeof x === 'string' && x.length > 0
回答by jukempff
To check against nullAND undefinedAND "empty string" you can just write
要检查nullAND undefinedAND “空字符串”，您可以只写
if(!x) {
// will be false for undefined, null and length=0
}
BUT you need to be sure that your variable is defined! Otherwise this will cause an error.
但是您需要确保您的变量已定义！否则这将导致错误。
If you are checking for a value in an object(e.g. the windowobject), you can always use that. E.g. for checking for localStoragesupport:
如果您正在检查object（例如window对象）中的值，您始终可以使用它。例如检查localStorage支持：
var supports = {
localStorage: !!window.localStorage
}
回答by iJade
Try this
试试这个
if (!x) {
// is emtpy
}
