Calling a javascript function from another .js file

The problem is, that menuHoverStart is not accessible outside of its scope (which is defined by the .ready() callback function in file #1). You need to make this function available in the global scope (or through any object that is available in the global scope):

function menuHoverStart(element, topshift, thumbchange) {
    // ...
}

$(document).ready(function() {
    // ...
});

If you want menuHoverStart to stay in the .ready() callback, you need to add the function to the global object manually (using a function expression):

$(document).ready(function() {
    window.menuHoverStart = function (element, topshift, thumbchange) {
        // ...
    };
    // ...
});