how to know at wich port server has started in frontend in heroku deploy code example

Example: vue js access heroku port

// vue.config.js
const webpack = require('webpack');
module.exports = {
  configureWebpack: {
    plugins: [
      new webpack.DefinePlugin({
        'process.env': {
          'PORT': JSON.stringify(process.env.PORT)
        }
      })
    ]
  }
}

// You can now access process.env.PORT anywhere in your vue application