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DOM allocation example
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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.