---
title: "Async scripts for asm.js - Game development | MDN"
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source: "https://developer.mozilla.org/en-US/docs/Games/Techniques/Async_scripts"
category: "reference"
tags: "web, html, css, javascript, documentation"
date_saved: "2026-05-05T05:21:38.111979+00:00"
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  3. [Async scripts for asm.js](https://developer.mozilla.org/en-US/docs/Games/Techniques/Async_scripts)


# Async scripts for asm.js
Every medium or large game should compile [asm.js](https://developer.mozilla.org/en-US/docs/Games/Tools/asm.js) code as part of an async script to give the browser the maximum flexibility to optimize the compilation process. In Gecko, async compilation allows the JavaScript engine to compile the asm.js off the main thread when the game is loading and cache the generated machine code so that the game doesn't need to be compiled on subsequent loads (starting in Firefox 28). To see the difference, toggle `javascript.options.parallel_parsing` in `about:config`.
## In this article
  * [Putting async into action](https://developer.mozilla.org/en-US/docs/Games/Techniques/Async_scripts#putting_async_into_action)
  * [When is async not async?](https://developer.mozilla.org/en-US/docs/Games/Techniques/Async_scripts#when_is_async_not_async)


##  [Putting async into action](https://developer.mozilla.org/en-US/docs/Games/Techniques/Async_scripts#putting_async_into_action)
Getting async compilation is easy: when writing your JavaScript, just use the `async` attribute like so:
or, to do the same thing via script:
(Scripts created from script default to `async`.) The default HTML shell Emscripten generates produces the latter.
##  [When is async not async?](https://developer.mozilla.org/en-US/docs/Games/Techniques/Async_scripts#when_is_async_not_async)
Two common situations in which a script is _not_ async (as [defined by the HTML spec](https://html.spec.whatwg.org/multipage/scripting.html)) are:
and
Both are counted as 'inline' scripts and get compiled and then run immediately.
What if your code is in a JS string? Instead of using `eval` or `innerHTML`, both of which trigger synchronous compilation, you should use a Blob with an object URL:
The setting of `src` rather than `innerHTML` is what makes this script async.
##  Help improve MDN 
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This page was last modified on Jul 11, 2025 by [MDN contributors](https://developer.mozilla.org/en-US/docs/Games/Techniques/Async_scripts/contributors.txt). 
[View this page on GitHub](https://github.com/mdn/content/blob/main/files/en-us/games/techniques/async_scripts/index.md?plain=1 "Folder: en-us/games/techniques/async_scripts \(Opens in a new tab\)") • [Report a problem with this content](https://github.com/mdn/content/issues/new?template=page-report.yml&mdn-url=https%3A%2F%2Fdeveloper.mozilla.org%2Fen-US%2Fdocs%2FGames%2FTechniques%2FAsync_scripts&metadata=%3C%21--+Do+not+make+changes+below+this+line+--%3E%0A%3Cdetails%3E%0A%3Csummary%3EPage+report+details%3C%2Fsummary%3E%0A%0A*+Folder%3A+%60en-us%2Fgames%2Ftechniques%2Fasync_scripts%60%0A*+MDN+URL%3A+https%3A%2F%2Fdeveloper.mozilla.org%2Fen-US%2Fdocs%2FGames%2FTechniques%2FAsync_scripts%0A*+GitHub+URL%3A+https%3A%2F%2Fgithub.com%2Fmdn%2Fcontent%2Fblob%2Fmain%2Ffiles%2Fen-us%2Fgames%2Ftechniques%2Fasync_scripts%2Findex.md%0A*+Last+commit%3A+https%3A%2F%2Fgithub.com%2Fmdn%2Fcontent%2Fcommit%2F21addd31954b2629ab3e186dacdf7edca813dc7d%0A*+Document+last+modified%3A+2025-07-11T18%3A40%3A52.000Z%0A%0A%3C%2Fdetails%3E "This will take you to GitHub to file a new issue.")
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