Time limited call to redis

timeLimitedRedis.js

/**
 * returns a function that acts like the Redis command indicated by cmd except that it will time out after a given number of milliseconds
 * 
 * @param {string} cmd The redis commmand to execute ('get','hset','sort', etc.)
 * @param {integer} timeLimit The number of milliseconds to wait until returning an error to the callback.
 * 
 */
function timeLimited(cmd, timeLimit) {
  //We aren't defining arguments here - this will allow the flexibility of passing a variable number of arguments to Redis
  return function() { //look ma, no explicit arguments
    var
      //arguments, even if not defined in the function declaration, are still stored in the arguments pseudo-array.
      //We'll first have to convert the arguments into a real array using slice - this will allow us to push/pop/apply to it
      argsAsArr = Array.prototype.slice.call(arguments),
      //assume that the last argument of any given call is a callback
      cb        = argsAsArr.pop(),
      //we'll hold the reference to the timeout in this variable
      timeoutHandler;
    
    //after the timeLimit, throw an error
    timeoutHandler = setTimeout(function(){
      cb(new Error('Redis timed out'));
      //make cb a no-op to prevent double callback
      cb = function() {};
    }, timeLimit);
    
    //since we've pop'ed off the original cb - let's add in our own replacement callback
    argsAsArr.push(function(err, values){
      //disable the original timeout
      clearTimeout(timeoutHandler);
      //call back as normal
      cb(err,values);
    });
    
    //send the original arguments, with the replaced callback
    client[cmd].apply(client,argsAsArr);
  };
}