To be fair, there's a bound how much the JIT/interpreter can do for a given language. Lua is a lot simpler, and a lot easier to optimize as a result, compared to JS.
If you squint a bit, Lua and Javascript are basically the same language. Lua shares many of javascript's peculiarities and warts (strings you can do arithmetic on, all numbers are doubles (originally), arrays are objects are hashtables of sorts, global scope by default etc). Now it's of course true that Lua is less of a mess than javascript, but a) the complexity explosion of javascript postdates V8 quite a bit b) lua has plenty of optimization challenges of its own that javascript lacks (e.g. metatables) c) the relative amount of resources poured into both projects differ by orders of magnitude (3?). I'm pretty doubtful anyone tasked to build the fastest JIT in the world from scratch upon being offered a choice between targetting lua (with luajit's total budget) and javascript (with v8's total budget) would pick lua.
No offence, but have you written any compilers or interpreters? The points that you discuss (all numbers are doubles, strings have arithmetic methods) may be performance concerns for application developers, but they have very little to do with the optimisations you can make as a compiler/interpreter writer.
The only one that's somewhat relevant is 'global scope by default', but this doesn't touch the surface of the issues that make JS hard to optimise, such as the fact that your, say, memoisation of an object property or method may be broken by an `eval` call of an arbitrary runtime value somewhere else in the code (which, due to asynchronicity, could take place at more or less any time from the point of view of your given 'peephole').
> No offence, but have you written any compilers or interpreters?
I have, but nothing sophisticated.
> The points that you discuss [...] may be performance concerns for application developers [...] but they have very little to do with the optimisations you can make as a compiler/interpreter writer. [...] The only one that's somewhat relevant is 'global scope by default'
I didn't mean to imply that these where the three common traits that make both Javascript and Lua particularly hard to optimize, I just picked them as examples for how Javascript and Lua are closer to each other than most other dynamic languages.
But let's dig in a bit on your claim that things like all numbers being doubles or having a array cum map cum record type has very little to do with the optimizations you can make as a compiler/interpreter writer, because it sure seems to me that LuaJIT and V8 do a bunch of optimizations around these things. Both have dual number representations under the hood and will try to avoid representing numbers that remain in the domain of 32 bit integers as double values internally when that gives performance gains. The logic for figuring out if that's the case doesn't seem to be super-straightforward or target architecture independent from looking at the comments in <https://github.com/LuaDist/luajit/blob/master/src/lj_opt_nar...>.
LuaJIT furthermore uses NaN tagging (as do some JS engines, although not V8), which looks less attractive to me as a representation strategy if your numbers are not all/mostly notional doubles (as is indeed the case in newer version of Lua where 64bit integers are the dominant number type). Do you disagree?
Also, as far as the super-flexible lua tables are concerned, I'm pretty sure LuaJIT goes through some amount of trouble to specialize various common use cases of tables, e.g. as arrays without holes, and surprise, so does V8 (https://v8.dev/blog/fast-properties#elements-or-array-indexe...). I don't think you'd find something equivalent in a high performance scheme implementation.
> but this doesn't touch the surface of the issues that make JS hard to optimise, such as the fact that your, say, memoisation of an object property or method may be broken by an `eval` call of an arbitrary runtime value somewhere else in the code (which, due to asynchronicity, could take place at more or less any time from the point of view of your given 'peephole').
Eval belongs to a core set of features that basically all popular dynamic languages share that presents headaches for high performance implementations. How is Javascript's eval particularly problematic in this regard, and specifically much more so than Lua's loadstring/load?
More generally what do you think makes (pre-ES6) javascript significantly harder to optimize than lua 5.0?
