javascript - 如何检查JavaScript中的空值？(How do I check for null values in JavaScript?)

Question

How can I check for null values in JavaScript?

(如何在JavaScript中检查空值？)

I wrote the code below but it didn't work.

(我在下面编写了代码，但是没有用。)

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

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

}   

And how can I find errors in my JavaScript programs?

(在JavaScript程序中如何找到错误？)

  ask by Mahdi_Nine translate from so

Answer 1

Javascript is very flexible with regards to checking for "null" values.

(Javascript在检查“空”值方面非常灵活。)

I'm guessing you're actually looking for empty strings, in which case this simpler code will work:

(我猜您实际上是在寻找空字符串，在这种情况下，这种简单的代码将起作用：)

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

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

(它将检查空字符串（ "" ）， null ， undefined ， false以及数字0和NaN)

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 , eg indexOf )

(请注意，如果您专门检查数字，则使用此方法会遗漏0是一个常见错误，并且num !== 0是首选（或num !== -1或~num （同样会检查-1 hacky代码） ））返回-1函数，例如indexOf ）)