This worked for me and excel document got uploaded in document library without being downloaded.

Here is the full code:

        function createAndUploadFile()
        {

        var web = context.get_web(); //gets the web object
        var list = web.get_lists(); //gets the list collection object
        var targetList;
        var fileCreation;
        var fileContent;
        targetList = list.getByTitle("Documents");
        fileCreation = new SP.FileCreationInformation();
        var d = new Date()
        var utc = d.toJSON().slice(0, 10) + "-" + d.getHours() + "-" + d.getMinutes() + "-" + d.getSeconds();
        var fileName = "EmployeeList" + utc; //gets the file name
        fileName = fileName + ".xls";
        fileCreation.set_url(fileName);
        fileCreation.set_content(new SP.Base64EncodedByteArray());
        fileContent = str;
        //gets the file contents.
        for (var content = 0; content < fileContent.length; content++) {
        fileCreation.get_content().append(fileContent.charCodeAt(content)); //add the text content to the file.
        }

        //create a new file instance
        var newFile = targetList.get_rootFolder().get_files().add(fileCreation);
        context.load(newFile);
        context.executeQueryAsync(onFileCreationSuccess, onFileCreationFail);

        }

        function onFileCreationSuccess()
        {
        alert("File created and uploaded in document library");
        }

        function onFileCreationFail()
        {
        alert("Error in creating file");
        }

You can call this function on page load or wherever you want.

This worked for me and excel document got uploaded in document library without being downloaded.