Change variable in function and keep it javascript?

Question

How can I write this so that lfmuser keeps the change made in the function called by UserModel.find()? I'd really rather not have to move a very VERY large section of code into each block if I can avoid it.

var np_handler = function (act) {
  var lfmuser = '';
  if (act.params.length === 0) {
    UserModel.find({ nick: act.nick }, function (err, data) {
      if (!data) {
        lfmuser = act.nick;
      } else {
        lfmuser = data.lastfm;
      }
    });
  } else {
    UserModel.find({ nick: act.params[0] }, function (err, data) {
      if (!data) {
        lfmuser = act.params[0];
      } else {
        lfmuser = data.lastfm;
      }
    });
  }
};

Ended up not mattering in this situation, I just moved the rest of the code (where I was using lfmuser) to it's own function np(lfm, act){ } and passed the right value when I called it. Async can be a pain :/

rjz · Accepted Answer · 2012-03-12 05:37:38Z

2

The easiest solution would be to move it outside of your np_handler function. Note that subsequent calls to the function will then overwrite its value, however.

answered Mar 12, 2012 at 5:37

rjz

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2 Comments

rjz Over a year ago

Nearly always. In Javascript, however, the structure of the application outside of the handling function will determine whether or not it's actually a global. If the handler is contained within some other closure, it may be appropriate (expedient) to declare the variable within that function.

Rob Over a year ago

Good point, I guess. It doesn't really matter what way I do it right now, because it's async and I need it not to be. It takes longer to grab the data than it does to try to use it. I guess I could move the rest of the code to a function and do it completely differently, I don't think I have a choice, really.

jamesmortensen · Accepted Answer · 2012-03-12 05:38:11Z

I created a wrapper object and listed your lfmuser variable as a property of the object. It's not global, and its value will persist after your function completes execution.

var np_handler_object = {
    lfmuser = '',  // variable in scope of the np_handler object and persists

    np_handler: function (act) {

        if (act.params.length === 0) {
            UserModel.find({ nick: act.nick }, function (err, data) {
                if (!data) {
                    np_handler_object.lfmuser = act.nick;
                } else {
                    np_handler_object.lfmuser = data.lastfm;
                }
            });
        } else {
            UserModel.find({ nick: act.params[0] }, function (err, data) {
                if (!data) {
                    np_handler_object.lfmuser = act.params[0];
                } else {
                    np_handler_object.lfmuser = data.lastfm;
                }
            });
        }
    };
}

ndp · Accepted Answer · 2012-03-12 05:53:20Z

The way you have it set up, the variable is scoped to np_handler. This means every time this function is called, you will get a fresh value (initialized to '').

It sounds like what you want is something that persists beyond this function, so in Javascript this is easy-- there is only function or global scope. So removing teh var lfmuser = '' line should do what you want.

BTW, while you are at it, a local variable and use of "?:" shortens the code, eg:

var lfmuser = '';
...

var np_handler = function (act) {
  var nick = act.params.length ? act.params[0] : act.nick;
  UserModel.find({ nick: nick }, function (err, data) {
    lfmuser = data ? data.lastfm : nick;
  });
};

If you don't want to use a global variable, you can scope the variable elsewhere, as others have pointed out.

jfriend00 · Accepted Answer · 2012-03-12 05:58:29Z

There are a number of ways to make a variable persistent across function calls. They all involve moving the variable declaration or storage outside of the scope of the function so that it doesn't get recreated and then die each time the function runs.

Property of the Function

One of the simpler ways is to just assign it as a property of the np_handler() function. This prevents any global namespace pollution or conflict, but makes it persist as long as the function that uses it exists. That can be done like this:

var np_handler = function (act) {
  if (act.params.length === 0) {
    UserModel.find({ nick: act.nick }, function (err, data) {
      if (!data) {
        np_handler.lfmuser = act.nick;
      } else {
        np_handler.lfmuser = data.lastfm;
      }
    });
  } else {
    UserModel.find({ nick: act.params[0] }, function (err, data) {
      if (!data) {
        np_handler.lfmuser = act.params[0];
      } else {
        np_handler.lfmuser = data.lastfm;
      }
    });
  }
};

// initialize
np_handler.lfmuser = '';

Global Scope

If you wanted to put it into the same scope as the function, you can just declare it at the that same scope and not declare it locally like this, though if np_handler was in the global scope, you now have one more item in the global scope which you generally try to avoid as much as possible:

var lfmuser = '';

var np_handler = function (act) {
  if (act.params.length === 0) {
    UserModel.find({ nick: act.nick }, function (err, data) {
      if (!data) {
        lfmuser = act.nick;
      } else {
        lfmuser = data.lastfm;
      }
    });
  } else {
    UserModel.find({ nick: act.params[0] }, function (err, data) {
      if (!data) {
        lfmuser = act.params[0];
      } else {
        lfmuser = data.lastfm;
      }
    });
  }
};

Global Namespace Object

If you need it to be globally accessible, but you don't want to pollute the global namespace any more than required or you want to avoid the possibility of a name conflict, you can create one global namespace object and make the global variable be a property of that object. You can then put all your globals as properties on the one master global object and only have introduced one new name in the global space rather than many. You would do that like this:

// make sure myGlobals is defined and assign a property to it
var myGlobals = myGlobals || {};
myGlobals.lfmuser = '';

var np_handler = function (act) {
  if (act.params.length === 0) {
    UserModel.find({ nick: act.nick }, function (err, data) {
      if (!data) {
        myGlobals.lfmuser = act.nick;
      } else {
        myGlobals.lfmuser = data.lastfm;
      }
    });
  } else {
    UserModel.find({ nick: act.params[0] }, function (err, data) {
      if (!data) {
        myGlobals.lfmuser = act.params[0];
      } else {
        myGlobals.lfmuser = data.lastfm;
      }
    });
  }
};

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