How to navigate between a root folder to a folder with a specific title with JSOM

Question

I want to navigate into a folder in SharePoint and create there a File. I can already create a file on the "root" folder of a selected list.

However I can't "switch" into a specific folder (document set) which is in the root folder.

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


folderPath = getParameterByName('RootFolder');
var folderPathArray = folderPath.split('/');
var documentSetName = folderPathArray[folderPathArray.length -1];

var encodedFolderPath = [];
folderPathArray.forEach(function (element) {
    encodedFolderPath.push(encodeURIComponent(element));
});
folderPath = encodedFolderPath.join("/");


fileCreateInfo = new SP.FileCreationInformation();
fileCreateInfo.set_url(folderPath + title + ".aspx");
fileCreateInfo.set_content(new SP.Base64EncodedByteArray());

fileContent = "helloworld";

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

// how to call get_files() in a document set so I can use the add method like below?
this.newFile = oList.get_rootFolder().get_files().add(fileCreateInfo);

var realFolder = oList.get_rootFolder().getByTitle("Special Folder");


clientContext.load(this.newFile);

clientContext.executeQueryAsync(onsuccess, onfailedownError');