chrome和firefox上的javascript null比较有所不同

Question

I have the following code:

var a=sessionStorage.getItem("Token");  
alert(a==null);

The returned value is null (If I alert(a) it displays null ). The problem is that the alert(a==null) display is TRUE on firefox and FALSE on safari and chrome. WTH? I have tried a===null with the same results as well as !a.

What am I doing wrong or what am I not aware of?

Thanks for any help.

Answer 1

You said in a comment: "I set Token with sessionStorage.setItem("Token",null); "

I believe the problem is that you are supposed to use session storage to store strings. When you pass null to setItem() it converts it to a string "null" . Then when you retrieve it with getItem() you get back this string "null" which is of course not equal to an actual null value.

You can see this behaviour here: http://jsfiddle.net/CWVww/1/

If you want to remove a previously set item then do this:

sessionStorage.removeItem("Token");

...and then calls to .getItem("Token") will return null .

I don't know why Firefox behaved differently. From the MDN page on session storage : "Keep in mind that everything you store in any of the storages described in this page is converted to string using its .toString method before being stored."

Answer 2

Your code worked perfectly with me (tested on Chrome). However, I suggest you to use the ! operator and also check the type of the current value:

var a = sessionStorage.getItem("Token");
if(!a && typeof a!=='string'){        //a doesn't exist
    //Do something
}else{        //a does exist
    //Do something
}

The operator ! will return true either when a is null or undefined .

Answer 3

You could try String(a) == "null" . However, if the value of the Token item is set to "null" (the string "null" ) the code won't work as expected, so we have to add another condition:

var a = sessionStorage.getItem("Token");
if(String(a)==="null" && typeof a!=="string"){        //a doesn't exist
    //Do something
}else{        //a does exist
    //Do something
}

This way, the condition will return true when the "stringified" value of a is "null" and the type of the a var is not string