提示js typeof的问题

Question

I have problems with check prompt data. I need to check, if the prompt data will be string, paragraph could show that data is not number. But according to my code, when I enter string data, it shows me odd or even message, but not 'Not number'. What's can be wrong? Thanks a lot!

Answer 1

prompt() always returns a string , use parseInt(prompt(), 10) to convert it to a string (10 is the numeric base, eg.: 2 means its a binary number)

It will return either a number or a NaN ( N ot A N umber) value.

typeof(NaN) === 'number'

NaN === NaN will result in false, use Number.isNaN to check if the value of a variable is NaN

if (!(a === b)) is the same as if (a !== b)

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Please, next time post your code as text instead of a sharing print screen of it, so we can ctrl+c, ctrl+v it

Answer 2

Because your second if condition evaluates as:

 !("nonsense" % 2 === 0)
 !(NaN % 2 === 0)
 !(NaN === 0)
 !(false)
 true

therefore it will always show Odd" for non numbers. Maybe you should validate your data before you use it. Additionally val will always be of type "string", you might want to parse it properly:

 const num = parseInt(prompt("A number?"), 10);
 if(isNaN(num)) {
   //...
 }