How can I get the local Storage Value

Your code should be :

var cookieValue = document.getElementById("demo");      
    var value = cookieValue .getAttribute('value');
    if(typeof(Storage)!=="undefined")
    {
        alert(value);
        localStorage.setItem("GetData" , value);
        alert(localStorage.getItem("GetData"));

    }

function loading()
{
        alert("coming");
        var allcookies = localStorage.getItem('GetData');
        alert(allcookies);

}

OR

var cookieValue = document.getElementById("demo");      
    var value = cookieValue .getAttribute('value');
    if(typeof(Storage)!=="undefined")
    {
        alert(value);
        localStorage["GetData"] = value;
        alert(localStorage["GetData"]);

    }

function loading()
{
        alert("coming");
        var allcookies = localStorage["GetData"];
        alert(allcookies);

}

Try this

if( window.localStorage ) {
    alert(value);
    localStorage.setItem("GetData" , value);
}

The correct syntax for setItem is

localStorage.setItem("GetData", value)

not

localStorage.setItem = ("GetData", value)