Edit in JSFiddle

$(function() {
    $("#loadScript").click(function() {
// getScript is an ajax request to download and execute the script
        $.getScript("http://cloud.github.com/downloads/SteveSanderson/knockout/knockout-2.1.0.js", function(data, textStatus) {
            alert(textStatus);
        });
    });

    // catch error in case the request fails
    $("#loadScript").ajaxError(function(event, jqXHR, ajaxSettings, thrownError) {
        alert("Error: " + thrownError);
    });
});

<button id="loadScript">Dynamically load and execute script</button>