======================
DOM allocation example
======================

This article describes a very simple web page that we'll use to illustrate some features of the Memory tool.

You can try out the site at https://firefox-devtools.github.io/performance-scenarios/dom-allocs/alloc.html.

It just contains a script that creates a large number of DOM nodes:

.. code-block:: javascript

  var toolbarButtonCount = 20;
  var toolbarCount = 200;

  function getRandomInt(min, max) {
      return Math.floor(Math.random() * (max - min + 1)) + min;
  }

  function createToolbarButton() {
    var toolbarButton = document.createElement("span");
    toolbarButton.classList.add("toolbarbutton");
    // stop Spidermonkey from sharing instances
    toolbarButton[getRandomInt(0,5000)] = "foo";
    return toolbarButton;
  }

  function createToolbar() {
    var toolbar = document.createElement("div");
    // stop Spidermonkey from sharing instances
    toolbar[getRandomInt(0,5000)] = "foo";
    for (var i = 0; i < toolbarButtonCount; i++) {
      var toolbarButton = createToolbarButton();
      toolbar.appendChild(toolbarButton);
    }
    return toolbar;
  }

  function createToolbars() {
    var container = document.getElementById("container");
    for (var i = 0; i < toolbarCount; i++) {
      var toolbar = createToolbar();
      container.appendChild(toolbar);
    }
  }

  createToolbars();

A simple pseudocode representation of how this code operates looks like this:

.. code-block:: JavaScript

  createToolbars()
    -> createToolbar() // called 200 times, creates 1 DIV element each time
       -> createToolbarButton() // called 20 times per toolbar, creates 1 SPAN element each time</pre>

In total, then, it creates 200 `HTMLDivElement <https://developer.mozilla.org/en-US/docs/Web/API/HTMLDivElement>`_ objects, and 4000 `HTMLSpanElement <https://developer.mozilla.org/en-US/docs/Web/API/HTMLSpanElement>`_ objects.
