SP.RequestExecutor on SharePoint online

Question

I deployed an App SharePoint hosted.
I am using SP.RequestExecutor and it is working fine on SharePoint premise.
but on SharePoint online it is not working.
I added the library using this: $.getScript("/_layouts/15/SP.RequestExecutor.js");
Can I use that library also in SharePoint online?

This is my CODE :

var createitem = new SP.RequestExecutor("/");
createitem.executeAsync({
    url: _spPageContextInfo.webAbsoluteUrl + "/_api/web/lists/GetByTitle('MyList')/items('" + id + "')/AttachmentFiles/add(FileName='" + file.name + "')",
    method: "POST",
    binaryStringRequestBody: true,
    body: contents2,
    success: function (data) {
        //OK
    },
    error: function (msg) {
        alert("error");
    },
    state: "Update"
});

Answer 1

I used this when I first created my app. First, you need both the app web url and the host web url:

var hostweburl = decodeURIComponent( getQueryStringParameter("SPHostUrl") );
var appweburl = decodeURIComponent( getQueryStringParameter("SPAppWebUrl") );

Then you need to load the SP.RequestExecutor.JS file, which I did differently than what was in the link:

var scriptbase = hostweburl + "/_layouts/15/";

$.getScript(scriptbase + "SP.Runtime.js",
    function () {
        $.getScript(scriptbase + "SP.js",
            function () { $.getScript(scriptbase + "SP.RequestExecutor.js", execCrossDomainRequest); }
        );
    }
);

execCrossDomainRequest is the function in which I executed the cross domain request. In my function, I was retrieving the five most recent items from a list called "Communication Log":

function execCrossDomainRequest() {
    var context;
    var factory;
    var appContextSite;

    context = new SP.ClientContext(appweburl);
    factory = new SP.ProxyWebRequestExecutorFactory(appweburl);
    context.set_webRequestExecutorFactory(factory);
    appContextSite = new SP.AppContextSite(context, hostweburl);

    var executor = new SP.RequestExecutor(appweburl);

    executor.executeAsync(
        {
            url:
                appweburl +
                    "/_api/SP.AppContextSite(@target)/web/lists/getbytitle('Communication%20Log')/items?$select=Title,Date1&$orderby=Date1 desc&$top=5&@target='" +
                        encodeURIComponent(hostweburl) + "'",
            method: "GET",
            headers: { "Accept": "application/json; odata=verbose" },
            success: successHandler,
            error: errorHandler
        }
    );
}

I'm sure you can piece the rest together. This was done when I was fairly fresh with both SharePoint apps and javascript, so it may be improved.