ES2015 / 2016'typeof varName ==='undefined`的方式？ [英] ES2015/2016 way of 'typeof varName === 'undefined`?

问题描述

我现在正在沉溺于ES2015 +奢侈品和一些项目，我想知道我是否可以摆脱那些讨厌的拐杖来检查新仙境中的 undefined 。

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 undefined in the new wonderland.

ES2015 +中是否有更短但仍然准确的 typeof varName ==='undefined'？

Is there a shorter but still exact way to typeof varName === 'undefined' in ES2015+ already?

当然我可以使用默认参数但这也是一种不必要的分配。

Of course I could use default parameters but this also feels like an unnecessary assignment.

function coolFn(a = null){
   if (a===null) console.log("no a supplied");
}

推荐答案

只需检查 varName === undefined 。

在旧浏览器中，可以为全局 undefined 变量导致该测试失败，但在ES2015 +中现在是不可能的。

In older browsers it was possible to assign an alternate value to the global undefined variable causing that test to fail, but in ES2015+ that's now impossible.

注意，没有办法区分显式传递 undefined 作为参数完全离开参数而不是通过查看 arguments.length 。

Note that there's no way to distinguish explicitly passing undefined as a parameter from leaving the parameter out altogether other than by looking at arguments.length.

这篇关于ES2015 / 2016'typeof varName ==='undefined`的方式？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！