Multer keeps giving me undefined

I'm looking to use Multer to upload an image file onto my node server. The code is simple, I copied it from Multer's docs. I spent the whole day stackoverflowing this issue and it always comes back to giving me undefined for using ajax to send the image.

Using a form DOES work: multer - req.file always undefined

However, I'm curious to see if anyone have successfully used ajax way to sending files.

HTML

<input type="file" id="avatar" name="avatar"/>

When the user clicks on the button and chooses a file, I simply display the file info, and use ajax to send the data to my node router http://localhost:8882/profile

js

<script>
//files structure contains data about each file you load in
   document.getElementById('avatar').addEventListener('change', function() {

    var file = this.files[0];
    console.dir(this.files[0]);

    postItem(this.files[0]);

}, false);


function postItem(imageObj) {
  console.log("postItem <----");
    $.ajax({
        type: "POST",
        url: "http://localhost:8882/profile",
        contentType:false,
        cache: false,
        processData: false,
        data:  imageObj,
        success: function(data) {
            //show content
            alert('Success!');
        },
        error: function(jqXHR, textStatus, err) {
            //show error message
            alert('text status ' + textStatus + ', err: '+err);
        }

    }); //ajax
}//postItem()

</script>

On the Node side, the code is directly from multer's docs: https://www.npmjs.com/package/multer

I keep getting undefined for req.file.

Node

var express     = require('express');
var app         = express();
var bodyParser  = require('body-parser');
var multer  = require('multer')
var upload = multer({ dest: 'uploads/' })

app.post('/profile', upload.single('avatar'), function (req, res, next) {
    console.log("------ /profile --------");

    if(req) {
        console.dir(req.file);
    }

    console.log("------------------------");
    res.send({"message":"ok"});
});

app.listen(8882);
console.log("index.js - listening on port " + 8882);
exports.app = app;

Would appreciate any help.