doc/docs/en/shared/optimization-memory-html5.md

Heap size (HTML5)

The heap size of a Defold HTML5 game can be configured from the heap_size field in game.project. Make sure to optimize memory usage of your game and set a minimal heap size.

For small games, 32 MB is an achievable heap size. For larger games, aim for 64–128 MB. If, for example, you're at 58 MB and further optimization isn't feasible, you can settle on 64 MB without overthinking it. There’s no strict target size — it depends on the game. Just aim for smaller sizes, ideally in steps of powers of two.

To check current heap usage you can launch your game and play the game in the most "resource heavy" level or section and monitor memory usage:

if html5 then
    local mem = tonumber(html5.run("HEAP8.length") / 1024 / 1024)
    print(mem)
end

You can also open the developer tools of your browser and write the following in the console:

HEAP8.length / 1024 / 1024

If the memory usage remains at 32 MB, that's great! If not, follow the steps to optimize the size of the engine itself and large assets such as sounds and textures.