Meteor - How to Trigger a Function when a Session Value Changes

How to Trigger a Function when a Session Value Changes

While working on a Meteor project, I wanted to trigger a function whenever I changed the value of a Session variable through Session.set(). It took me a little bit of experimentation and digging to figure out how to do it, and the result is pretty simple.

How to do it

Use the following code:

var doSomething = function() {
  // Do something when the session value changes
}

Tracker.autorun(function() {
  var sessionVal = Session.get("yourSessionVariable");
  console.log("The session value has changed");
  doSomething();
});

var anotherFunction = function() {
  Session.set("yourSessionVariable", "foo");
}

That’s it! Whenever anotherFunction is executed, or whenever the session value yourSessionVariable is changed in any other function, the function in the Tracker.autorun() block will execute. Simple!

Tracker.autoRun

Tracker.autoRun() is the key here, as it can make arbitrary blocks of code reactive. Session is already a reactive variable, so we do not need to go through the trouble of using Tracker.Dependency. You can find more information about Tracker here.