sessionStorage未按预期工作

Question

Here is my code:

    sessionStorage.loggedIn = true;

    if (sessionStorage.loggedIn) {
        alert('true');
    }
    else {
        alert('false');
    }

Simple enough. There must be some small thing I'm not understanding about how JavaScript is evaluating these expressions. When I put sessionStorage.loggedIn = false , the "false" alert shows correctly. However, when I change sessionStorage.loggedIn to true, the "false" alert still pops, even after clearing the session. What am I not getting right with this expression? It seems so simple, maybe I just need another pair of eyes on it.

Answer 1

Try to change your code to

sessionStorage.setItem('loggedIn',JSON.stringify(true));

if (JSON.parse(sessionStorage.getItem('loggedIn'))) {
    alert('true');
}
else {
    alert('false');
}

and it should work consistently across all major browsers.

The interface with the setItem / getItem methods is how the spec is written, so going that way is safer than using the shortcut of assigning properties. Also, sessionStorage , like localStorage is a textbased storage mechanism, and not meant for storing objects, so you need to wrap calls with JSON.parse and JSON.stringify to get the expected results across the board.

Be aware that JSON.parse doesn't always play nice with undefined/null values, so it might be wise to do some type checking first.

You can read the spec for the storage interface here

Answer 2

Keys and Values in a WebStorage object (sessionStorage) must be strings. If they are not strings they "should" be converted to strings in the browser's implementation when you assign to sessionStorage. If you evaluate against "true" or convert to boolean it will work fine.

https://code.google.com/p/sessionstorage/

http://www.w3schools.com/html/html5_webstorage.asp