Express node app.js not working

Below is my package.json. I added start in scripts after referring few answers

{
"name": "chat",
 "version": "0.0.1",
 "description": "chat",
 "dependencies": {
  "express": "^4.15.2",
  "socket.io": "^1.7.3"
 },
"main": "app.js",
"devDependencies": {},
"scripts": {
"test": "echo \"Error: no test specified\" && exit 1",
     "start": "node app.js"
 },
"author": "",
"license": "ISC"
}

Below is my app.js

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

app.get('/', function (req, res) {
res.send('Hello World!')
})

app.listen(3000, function () {
 console.log('Example app listening on port 3000!')
})

Kindly assist

via Mani