Javascript ES2015/2016 的 'typeof varName === 'undefined` 方式？
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ES2015/2016 way of 'typeof varName === 'undefined`?
提问by Hedge
I'm wallowing in ES2015+ luxury with a few projects right now and am wondering whether I can get rid of the much hated crutch to check for undefinedin the new wonderland.
我现在正在沉迷于 ES2015+ 的奢华和一些项目，我想知道我是否可以摆脱令人讨厌的拐杖来检查undefined新的仙境。
Is there a shorter but still exact way to typeof varName === 'undefined'in ES2015+ already?
typeof varName === 'undefined'在 ES2015+ 中是否有更短但仍然准确的方法？
Of course I could use default parametersbut this also feels like an unnecessary assignment.
当然我可以使用默认参数，但这也感觉像是一个不必要的分配。
function coolFn(a = null){
if (a===null) console.log("no a supplied");
}
回答by Alnitak
Just check for varName === undefined.
只需检查varName === undefined.
In older browsers it was possible to assign an alternate value to the global undefinedvariable causing that test to fail, but in ES2015+ that's now impossible.
在较旧的浏览器中，可以为全局undefined变量分配一个替代值，导致该测试失败，但在 ES2015+ 中，这是不可能的。
Note that there's no way to distinguish explicitly passing undefinedas a parameter from leaving the parameter out altogether other than by looking at arguments.length.
请注意，undefined除了查看arguments.length.
回答by Oriol
The only case where typeof varName === 'undefined'is useful is when you don't know whether the variable varNamehas been declared.
唯一typeof varName === 'undefined'有用的情况是您不知道变量varName是否已声明。
And IMO if you don't know whether your variables are declared, your code has serious problems.
如果你不知道你的变量是否被声明，你的代码就有严重的问题。
In other cases you have better options:
在其他情况下，您有更好的选择：
varName === void 0This will detect whether
varNameis undefined.voidis an operator which receives an argument (you can use whatever instead of0), and returns undefined.varName === undefinedThis shoulddetect whether
varNameis undefined.However, be aware the global
undefinedcould have been overwritten (before ES5) or shadowed with another value. Therefore I prefervoid, which is also shorter.varName == nullThis will detect whether
varNameis undefined or is null.!varNameThis will detect whether
varNameis falsy (undefined, null, 0, empty string, NaN, false).
varName === void 0这将检测是否
varName未定义。void是一个运算符，它接收一个参数（你可以使用任何东西代替0），并返回 undefined。varName === undefined这应该检测是否
varName未定义。但是，请注意全局
undefined可能已被覆盖（在 ES5 之前）或被另一个值遮蔽。因此我更喜欢void，它也更短。varName == null这将检测是否
varName未定义或为空。!varName这将检测是否
varName为假（未定义、空、0、空字符串、NaN、假）。
