Upload document to a specific folder in a Document Library using JavaScript

Question

I Want to upload document to the Document library's specific folder using JavaScript/JQuery. I tried with the below code. But I can upload documents to the root folder only not to the specific folder.

function sendFileToServer(library, filename, file) 
{
    var reader = new FileReader();
    reader.onloadend = function(evt) 
    {
      if (evt.target.readyState == FileReader.DONE) 
      {
        var serverRelativeUrlToFolder = '/AttachmentLib/Test';
         var buffer = evt.target.result;
         //Upload Files to the Root Folder
         //var completeUrl = String.format(
            //"//{0}/_api/Web/Lists/getByTitle('AttachmentLib')/RootFolder/Files/Add(url='{1//}', //overwrite=true)",
            //_spPageContextInfo.webAbsoluteUrl, filename);
            //Upload Files to the Specific folder
        var completeUrl = String.format(
                 "{0}/_api/web/getfolderbyserverrelativeurl('{1}')/files" +
                 "/add(overwrite=true, url='{2}')",
                 _spPageContextInfo.webAbsoluteUrl, serverRelativeUrlToFolder, filename);
          $.ajax({
                url: completeUrl,
                type: "POST",
                data: buffer,
                async: false,
                processData: false,
                headers: {
                    "accept": "application/json;odata=verbose",
                    "X-RequestDigest": $("#__REQUESTDIGEST").val()
                    //"content-length": buffer.byteLength
                },
                complete: function (data) {
                    alert('succes');
                },
                error: function (err) {
                     alert('failed');
                }
        });
      }
    };
    reader.readAsArrayBuffer(file);
}

Answer 1

Solved!!!
I done this with the below code

function sendFileToServer(filename, file) 
{
    var reader = new FileReader();
    reader.onloadend = function(evt) 
    {
        if (evt.target.readyState == FileReader.DONE) 
        {
            var folder = localStorage.getItem('FolderName');

            var serverRelativeUrlToFolder = 'LibraryName/' + folder;
            var buffer = evt.target.result;
            //Upload to Specific folder 
            var docLibraryEndpoint = String.format( "{0}/_api/web/getfolderbyserverrelativeurl('{1}')/files" +
                            "/add(overwrite=true,url='{2}')",
                            _spPageContextInfo.webAbsoluteUrl, serverRelativeUrlToFolder, filename
                            );
            $.ajax({
                url: docLibraryEndpoint,
                type: "POST",
                data: buffer,
                processData: false,
                headers: {
                    "accept": "application/json;odata=verbose",
                    "X-RequestDigest": $("#__REQUESTDIGEST").val()
                },
                success:function()
                {
                    alert("Document uploaded successfully");
                },
                error:function()
                {
                    alert("Failed to upload the document");
                }
            });
        }
    };
    reader.readAsArrayBuffer(file);
}