如何在 JavaScript 中检查 null 值?

How do I check for null values in JavaScript?(如何在 JavaScript 中检查 null 值?)

问题描述

如何在 JavaScript 中检查空值?我写了下面的代码，但是没有用.

How can I check for null values in JavaScript? I wrote the code below but it didn't work.

if (pass == null || cpass == null || email == null || cemail == null || user == null) {      

    alert("fill all columns");
    return false;  

}   

我如何在我的 JavaScript 程序中发现错误?

And how can I find errors in my JavaScript programs?

推荐答案

JavaScript 在检查null"方面非常灵活.价值观.我猜你实际上是在寻找空字符串，在这种情况下，这个更简单的代码会起作用:

JavaScript is very flexible with regards to checking for "null" values. I'm guessing you're actually looking for empty strings, in which case this simpler code will work:

if(!pass || !cpass || !email || !cemail || !user){

它将检查空字符串 ("")、null、undefined、false 和数字 0 和 NaN.

Which will check for empty strings (""), null, undefined, false and the numbers 0 and NaN.

请注意，如果您专门检查数字，使用此方法会错过 0 是一个常见错误，并且首选 num !== 0(或num !== -1 或 ~num(也检查 -1 的黑客代码))用于返回 -1 的函数，例如indexOf).

Please note that if you are specifically checking for numbers, it is a common mistake to miss 0 with this method, and num !== 0 is preferred (or num !== -1 or ~num (hacky code that also checks against -1)) for functions that return -1, e.g. indexOf).

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