testlocalstorage.js

function testLocalStorage() {

    let data = '';

    for(var key in window.localStorage) {
        if(window.localStorage.hasOwnProperty(key)){
            data = `${data}${window.localStorage[key]}`;
            console.log(`${key} = ${((window.localStorage[key].length * 16)/(8 * 1024)).toFixed(2)} KB`);
        }
    }

    console.log(data ? `\n Total space used:  ${((data.length * 16)/(8 * 1024)).toFixed(2)} KB` : 'Empty (0 KB)');
    console.log(data ? `Approx. space remaining: ${(5120 - ((data.length * 16)/(8 * 1024)).toFixed(2))} KB` : '5 MB');

    try {
        localStorage.setItem('localStorageTestItem', 'localStorageTestItem');
        console.log('localStorageTestItem written: ', window.localStorage['localStorageTestItem'])
    } catch (e) {
        console.log('localStorage setItem error: ', e);
    }

}

testLocalStorage()