Find a usable port by given port-range

findMyPort.js

var netstat = require("node-netstat");


var getNextAvailablePort = function (portRangeAsString) {
  var portString = portRangeAsString || process.env.USE_PORT_RANGE;

  if (portString) {
    var portStart, portLast;

    portStart = parseInt(portString.split("-")[0]);
    portLast = parseInt(portString.split("-")[1]);

    var portArr = [];

    netstat({
      sync: true,
      filter: {
        local: {
          address: null
        }
      }
    }, portArr.push.bind(portArr));

    portArr = portArr.map(
      portInfo => portInfo.local.port
    ).filter(
      // filter port range and also the index to eliminate duplicates
      (portNr, index, arr) => portNr >= portStart && portNr <= portLast && arr.indexOf(portNr) === index
    );
    
    if (portArr.length > portLast - portStart) {
      throw new Error("No available port in the range " + portString);
    }
    else {
      for(var i = portStart; i <= portLast; ++i) {
        if (portArr.indexOf(i) < 0) {
          return i;
        }
      }
    }
  }
  // node will decide whihc port to take
  else {
    return 0; 
  }
}

console.log(
  getNextAvailablePort()
);