How add contents to the exisiting text file using CSOM in a sharepoint HOSTED APP ?

Hi,

I am trying to write data in a existing text file , which present in my Site Collection Documents , The problem is whenever I try to write into that file the new data is written into the file , I want to add data to , means the previous data should not go.

Please suggest a way !

I am using this code :

function createFile(resultpanel) {
    var clientContext;
    var oWebsite;
    var oList;
    var fileCreateInfo;
    var fileContent;

    clientContext = new SP.ClientContext.get_current();
    oWebsite = clientContext.get_web();
    oList = oWebsite.get_lists().getByTitle("Shared Documents");

    fileCreateInfo = new SP.FileCreationInformation();
    fileCreateInfo.set_url("my new file.txt");
    fileCreateInfo.set_content(new SP.Base64EncodedByteArray());
    fileContent = "The content of my new file";

    for (var i = 0; i < fileContent.length; i++) {
       
        fileCreateInfo.get_content().append(fileContent.charCodeAt(i));
    }

    this.newFile = oList.get_rootFolder().get_files().add(fileCreateInfo);

    clientContext.load(this.newFile);
    clientContext.executeQueryAsync(
        Function.createDelegate(this, successHandler),
        Function.createDelegate(this, errorHandler)
    );

    function successHandler() {
        resultpanel.innerHTML =
            "Go to the "+
            "<a href='../Lists/Shared Documents'>document library</a> "+
            "to see your new file.";
    }

    function errorHandler() {
        resultpanel.innerHTML = "Request failed: "+ arguments[1].get_message();
    }
}