Conditionally executing a callback within an asynchronous function call in a for loop

Question

I'm working on a function that iterates through a list of ports and calls its callback once one of the ports is available. Instead of waiting for each port response, I'd like to call a checkPortStatus function on all the ports and return when the first port returns as available.

Here's sample code runnable in node.js:

    // Check if a port is open (in use) or closed (available)
    function checkPortStatus(port, callback) {
      // Simulates real world checking of response from port by returning
      // after a random time.
      setTimeout(function() {
        // Simulates a few closed ports
        if (port === 3005 || port === 3006 || port === 3007) {
          callback('closed')
        }
        // Simulates a few open ports
        else {
          callback('open')
        }
      }, Math.floor(Math.random() * 200))
    }

    // Calls callback once any available port is found
    function findAnAvailablePort(startPort, endPort, callback) {
      var check = function(port) {
        checkPortStatus(port, function(status) {
          // If a port is 'closed', it's considered available
          if (status === 'closed') {
            callback(port)
          }
        })
      }
      for (var port = startPort; port <= endPort; port++) {
        check(port)
      }
    }

    findAnAvailablePort(3000, 3010, function(port) {
      console.log('AVAILABLE PORT: ' + port)
    });

    // Don't exit Node, wait for a response
    setTimeout(function() { console.log('FIN') }, 5000)

Results of running the code:

AVAILABLE PORT: 3005
AVAILABLE PORT: 3007
AVAILABLE PORT: 3006

Two questions:

How do I call the callback only once? I want only the first port that matches to return.
How do I check if all function callbacks returned? In this example, I need to return when all checkPortStatus return ports as unavailable.

David Wick · Accepted Answer · 2011-08-11 23:43:44Z

1

initializing a few variables outside of the function should do the trick:

// Calls callback once any available port is found
var hasFoundPort = false;
var checkedPorts = 0;
function findAnAvailablePort(startPort, endPort, callback) {
  var check = function(port) {
    checkPortStatus(port, function(status) {
      checkedPorts++;
      // If a port is 'closed', it's considered available
      if (status === 'closed' && !hasFoundPort) { // only execute callback once
        callback(port)
        hasFoundPort = true;
      }
      if(checkedPorts == endPort - startPort + 1) // checked all ports
          console.log('finished checking');
    })
  }
  for (var port = startPort; port <= endPort; port++) {
    check(port)
  }
}

edited Aug 11, 2011 at 23:43

answered Aug 11, 2011 at 3:29

David Wick

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2 Comments

TiansHUo Over a year ago

You should also close the other ports when a port is already found.

baalexander Over a year ago

Thanks David, I was over-thinking the problem. Your solution worked. Code can be found here: github.com/baalexander/node-portscanner

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