What is the usage of the construct: function F() { if (!(this instanceof F)) { return new F() }; ... }?

I found this in a pty.js for Node. Here is the original code:

function Terminal(file, args, opt) {

  if (!(this instanceof Terminal)) {

     return new Terminal(file, args, opt);

  }



  var self = this

     , env

     , cwd

     , name

     , cols

     , rows

     , term;

-------------------SKIP-----------------------------------

  Terminal.total++;

  this.socket.on('close', function() {

     Terminal.total--;

     self._close();

     self.emit('exit', null);

  });



  env = null;

}