Question

I have given below simplified JavaScript problem.

var PROJ=(function(){
    var tags={}, 
    var lock=true;
    function onLoadComplete(){}
    this.Tag = function(userConfig,callBack){       
            function loadConfig(){
                    lock=false;             
                    /* Do something privately having lock with me */
                    lock=true;
            }                           
            this.load(){
                  if(lock)  loadConfig();
                  else      setTimeout(load,1000); 
              // PROBLEM is this(above) load is calling OUTER load..!
            }
       return this;
    };

    this.load(){
      var cb=onLoadComplete;
      tags[uniqueID]=new Tag(userConfig,cb);
      tags[uniqueID].load();
    }       
}).load();

I am trying to implement JavaScript locks here. Three JavaScript Tag objects are created. [ new Tag() ]These objects share and modify some public data available in PROJ.

I want to give access to public data when lock has been released by executing Tag. This load mentioned in setTimeout() is calling load() of PROJ.

Answer 1

I am presuming that the code actually looks like this:

this.Tag = function(userConfig,callBack){       
        function loadConfig(){
                lock=false;             
                /* Do something privately having lock with me */
                lock=true;
        }                           
        this.load = function(){
              if(lock)  loadConfig();
              else      setTimeout(load,1000); 
        }
   return this;
};

Since JavaScript is single threaded, it is not possible for the load function to be invoked while loadConfig is still running. This means that it is not possible for the lock variable to be false inside load, unless:

1. There is a return statement in loadConfig which returns execution before lock is set to true.
2. There is an exception thrown in the middle of the function, and the last line is never executed.

For the latter case, you can use the try/finally block to make sure that lock is reset in case of an exception:

        function loadConfig(){
            try {
                lock=false;             
                /* Do something privately having lock with me */
            }
            finally {
                // this will get executed even if an exception is thrown
                lock=true;
            }
        } 

The bottom line is, lock is not needed at all and can be removed completely. There is no way load could run concurrently with loadConfig.