你如何在 JavaScript 中实现一个保护子句？
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/5339121/
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
How do you implement a guard clause in JavaScript?
提问by luebken
I want to guard my functions against null-ish values and only continue if there is "defined" value.
我想保护我的函数免受空值的影响，并且只有在有“定义”值时才继续。
After lookingaroundthe solutions suggested to double equal to undefined: if (something == undefined). The problem with this solution is that you can declare an undefined variable.
之后找周围的解决方案，建议增加一倍等于未定义：if (something == undefined)。此解决方案的问题在于您可以声明未定义的变量。
So my current solution is to check for null if(something == null)which implicetly checks for undefined. And if I want to catch addionalty falsy values I check if(something).
所以我目前的解决方案是检查 nullif(something == null)隐式检查未定义。如果我想捕捉额外的虚假值，我会检查if(something).
See tests here: http://jsfiddle.net/AV47T/2/
在这里查看测试：http: //jsfiddle.net/AV47T/2/
Now am I missing something here?
现在我在这里错过了什么吗？
Matthias
马蒂亚斯
回答by Travis Webb
The standard JS guard is:
标准的 JS 守卫是：
if (!x) {
// throw error
}
!xwill catch any undefined, null, false, 0, or empty string.
!x将捕获任何undefined, null, false, 0, 或空字符串。
If you want to check if a value is valid, then you can do this:
如果你想检查一个值是否有效，那么你可以这样做：
if (Boolean(x)) {
// great success
}
In this piece, the block is executed if x is anything butundefined, null, false, 0, or empty string.
在这一块，如果x是任何执行该块，但undefined，null，false，0，或空字符串。
-tjw
-tjw
回答by Shadow Wizard is Ear For You
The only safe way that I know ofto guard against reallyundefined variables (meaning having variable name that were never defined anywhere) is check the typeof:
我所知道的防止真正未定义的变量（意味着从未在任何地方定义过变量名）的唯一安全方法是检查typeof：
if (typeof _someUndefinedVarName == "undefined") {
alert("undefined");
return;
}
Anything else (including if (!_someUndefinedVarName)) will fail.
其他任何东西（包括if (!_someUndefinedVarName)）都会失败。
Basic example: http://jsfiddle.net/yahavbr/Cg23P/
基本示例：http: //jsfiddle.net/yahavbr/Cg23P/
Remove the first block and you'll get:
删除第一个块，你会得到：
_someUndefinedVarName is not defined
_someUndefinedVarName 未定义
回答by Rich
Only recently discovered using '&&' as a guard operator in JS. No more If statements!
最近才发现在 JS 中使用“&&”作为保护运算符。没有更多的 If 语句！
var data = {
person: {
age: 22,
name: null
}
};
var name = data.person.name && doSomethingWithName(data.person.name);
回答by Arnaud Debray
Ternary to the rescue !
三元来救援！
(i) =>
i == 0 ? 1 :
i == 1 ? 2 :
i == 2 ? 3 :
null
