Check if localStorage is available

Use modernizr's approach (you might want to change my function name to something better):

function lsTest(){
    var test = 'test';
    try {
        localStorage.setItem(test, test);
        localStorage.removeItem(test);
        return true;
    } catch(e) {
        return false;
    }
}

if(lsTest() === true){
    // available
}else{
    // unavailable
}

It's not as concise as other methods but that's because it's designed to maximise compatibility.

The original source: https://github.com/Modernizr/Modernizr/blob/master/feature-detects/storage/localstorage.js

Working example: http://jsfiddle.net/6sm54/2/


I'd check that localStorage is defined prior to any action that depends on it:

if (typeof localStorage !== 'undefined') {
    var x = localStorage.getItem('mod');
} else {
    // localStorage not defined
}

UPDATE:

If you need to validate that the feature is there and that it is also not turned off, you have to use a safer approach. To be perfectly safe:

if (typeof localStorage !== 'undefined') {
    try {
        localStorage.setItem('feature_test', 'yes');
        if (localStorage.getItem('feature_test') === 'yes') {
            localStorage.removeItem('feature_test');
            // localStorage is enabled
        } else {
            // localStorage is disabled
        }
    } catch(e) {
        // localStorage is disabled
    }
} else {
    // localStorage is not available
}

Feature-detecting local storage is tricky. You need to actually reach into it. The reason for this is that Safari has chosen to offer a functional localStorage object when in private mode, but with it's quotum set to zero. This means that although all simple feature detects will pass, any calls to localStorage.setItem will throw an exception.

Mozilla's Developer Network entry on the Web Storage API's has a dedicated section on feature detecting local storage. Here is the method recommended on that page:

function storageAvailable(type) {
    try {
        var storage = window[type],
            x = '__storage_test__';
        storage.setItem(x, x);
        storage.removeItem(x);
        return true;
    }
    catch(e) {
        return false;
    }
}

And here is how you would use it:

if (storageAvailable('localStorage')) {
    // Yippee! We can use localStorage awesomeness
}
else {
    // Too bad, no localStorage for us
}

If you are using NPM, you can grab storage-available using

npm install -S storage-available

then use the function like so:

if (require('storage-available')('localStorage')) {
    // Yippee! We can use localStorage awesomeness
}

Disclaimer: Both the documentation section on MDN and the NPM package were authored by me.