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Exploring the Heap Contents

This page demonstrates how object contents can be explored using the Heap Profiler. The entire contents can be viewed, regardless of the current context (as opposed to how debugger works). Moreover, it is possible to inspect variables of closures, and peek into objects internals.

Below is the source code of the script, for reference:

function createClosure(a, b, c)
  var d = a + b;
  return function() { return c + d; };

var closure = createClosure("a", "b", "c");
closure.a = "property a";
closure.d = "property d";

var consString = "aaa / " + document.URL + document.inputEncoding;

var top_in_page = "inside page";

function init()
  var iframe = document.createElement("iframe");
  iframe.setAttribute("src", "heap-profiling-containment-files/iframe.html");

Try this:

  • Take a heap snapshot
  • Open the Containment view

The Containment view shows several top-level entries, including DOMWindow objects. By expanding a DOMWindow object, it is possible to examine all of its properties. This page contains several iframe, and it has its own DOMWindow object.

There are several interesting things to explore in the DOMWindow object of the page:

  • Find and expand the closure property. It will show the contents of the closure, as well as properties of the closure object itself. Color coding is used to distinguish closure variables from properties.
  • Find and expand the consString property. It will exhibit, that a concatenated string is stored as a linked list of its parts.

Simultaneously, you can view contents of an iframe without the need to switch contexts.