glasser

    If you are using Chrome, open the Developer Tools and click the gear
    icon in its lower right corner. In the General Settings panel, turn
    on 'Enable source maps'.
    
    If you are using Firefox 23, go to `about:config` and set the
    `devtools.debugger.source-maps-enabled` preference to true.
    (The preference should be on by default in Firefox 24; versions
    older than 23 do not support source maps.)

    var theThing = null;
    var replaceThing = function () {
      var originalThing = theThing;
      // Define a closure that references originalThing but doesn't ever actually
      // get called. But because this closure exists, originalThing will be in the
      // lexical environment for all closures defined in replaceThing, instead of
      // being optimized out of it. If you remove this function, there is no leak.
      var unused = function () {
        if (originalThing)
          console.log("hi");
      };
      theThing = {
        longStr: new Array(1000000).join('*'),
        // While originalThing is theoretically accessible by this function, it
        // obviously doesn't use it. But because originalThing is part of the
        // lexical environment, someMethod will hold a reference to originalThing,
        // and so even though we are replacing theThing with something that has no
        // effective way to reference the old value of theThing, the old value
        // will never get cleaned up!
        someMethod: function () {}
      };
      // If you add `originalThing = null` here, there is no leak.
    };
    setInterval(replaceThing, 1000);