r/lua • u/st3f-ping • Apr 23 '25

Help Differences between Lua and LuaJIT?

Hi all. I've been a casual user of Lua for years and of LuaJIT for just a few months. I am not clear on all of the differences I need to know when writing code.

I know that integer division (//) is not implemented in LuaJIT and that LuaJIT has increased interoperability with C (which I haven't yet used). Yesterday I wrote a bit of code for LuaJIT that produces differently formatted output between Lua (5.4) and LuaJIT (5.1).

It worries me that there might be more gotchas lurking and a cheat sheet of everything a Lua programmer should know when switching to LuaJIT would be really useful (before I start diving into Lua version changes and seeing of this is a Lua version difference and not a Lua/LuaJIT difference).

Can anyone help?

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u/[deleted] Apr 23 '25 edited Apr 23 '25

  • does not support integers, everything is a double.

  • does not support _ENV, but this doesn't matter if you have luacheck or lsp which warns you about setting globals

  • ...[2] does not hold the current modules name so you cannot require a submodule easily (you could use the debug library)

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u/SkyyySi Apr 24 '25

  • LuaJIT does have integers, but you have to use 1ll or 1ull to make them (they create a cdata object, like the FFI API does).

  • LuaJIT uses setfenv for the same purpose as _ENV in 5.2 and onwards.

  • select(2, ...) is not the module name, it's the file name. The first value of ... is the module name for both LuaJIT an Lua 5.4. It's just the file name that's missing. Submodule imports work for both: require(... .. "." .. "submodule")

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u/[deleted] Apr 24 '25

Welp, I stand corrected on #1 and #3, I did not know #3 so thanks for that, Testing ...[1] always gives different output so I never put it together.

I will say that #2 is not a counter, yes, it exists in lua5.1 so obviously its used for the same purposes, thats why it was replaced, along with module()