[whatwg] Proposal for separating script downloads and execution

Nicholas Zakas nzakas at yahoo-inc.com
Tue Feb 1 09:53:31 PST 2011

Problem Statement:

Loading JavaScript onto a page poses several performance issues. With a regular <script> tag, the UA waits for download and then waits for execution. The defer attribute helps by not blocking on download and deferring execution until later but preserves execution order; the async attribute helps by not blocking on download but does block on execution (the timing of which cannot be controlled) and does not preserve order.

Each of the existing solutions shifts around when download and execution happens by giving developers control over when the download occurs but only minimally when execution happens. As a result, developers have created ever more ingenius/fragile solutions to allow the separation of script downloads and execution. Examples:

1. Preloading JS without execution (http://www.phpied.com/preload-cssjavascript-without-execution/) by Stoyan Stefanov, which describes how to download JavaScript without execution it, as a cache-warming technique.
2. ControlJS (http://stevesouders.com/controljs/) by Steve Souders, which extends Stoyan's model to allow on-demand execution of scripts.
3. Gmail putting JavaScript in comments and then parsing later (http://googlecode.blogspot.com/2009/09/gmail-for-mobile-html5-series-reducing.html) to enable download without execution and then execution on-demand.

The ability to separate download and execution is a trend that has not only emerged, but continues to be explored. There are problems with the previous solutions, the biggest of which (in the case of #1 and #2) is the reliance on the browser caching behavior which may, in some instances, lead to a double download of the same script. It would be preferable for a standardized approach to achieve these goals.

Overview of Proposal:

Add a new attribute to the <script> called noexecute (for lack of a better term) that instructs the browser to download the script but do not execute it. Developers must manually execute the code by calling an execute() method on the script node. Simple example:

var script = document.createElement("script");
script.noexecute = true;
script.src = "foo.js";


Proposal Details:
Because there are a lot of nuanced changes to <script> as a result of this proposal, I've written up a full description here:

I'd love some feedback on whether or not this a) makes sense and b) is feasible.


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