Heroku Error: Web process failed to bind to $PORT within 60 seconds of launch

Getting this error in node.js but not sure why.

index.js lives in my root level app folder and looks like:

var express = require('express');
var app = express();

app.use('/', express.static('public'));

app.get('/', function(req, res) {
    res.sendFile(__dirname + '/index.html');
});

app.listen(process.env.PORT || 8080);

and when running heroku open I get an error:

2017-03-15T15:41:18.615808+00:00 heroku[web.1]: Error R10 (Boot timeout) -> Web process failed to bind to $PORT within 60 seconds of launch
2017-03-15T15:41:18.616112+00:00 heroku[web.1]: Stopping process with SIGKILL
2017-03-15T15:41:18.733009+00:00 heroku[web.1]: Process exited with status 137

Procfile looks like: web: node ./app/index.js

Can post more code but looking at other issues these seem like the solutions people have but in place..

via The worm