ajross is right. People choose the languages they like regardless of performance.
JS perf is important to users though. Faster execution means fewer watts spent rendering and interacting with your favorite web page.
(Fun fact: B3's backend contains a machine description language that gets compiled to C++ code by a ruby script, opcode_generator.rb. We use Ruby a lot.)
I wasn't really referring to self hosting. I was wondering whether it made sense to write tools like JavaScriptCore's offlineasm in JavaScript rather than Ruby (as LuaJIT does by using Lua for its dynasm tool). The LuaJIT build process actually builds a cut down copy for Lua for the purposes of running dynasm, which is then used to build parts of LuaJIT.
We spend time optimizing JavaScriptCore's build time. It's a hard problem and it's important when managing such a large project. People inevitably have to do clean builds and that sucks when you have >400k lines of C++ code.
So, naturally, we want to avoid building JSC twice. ;-)
JS perf is important to users though. Faster execution means fewer watts spent rendering and interacting with your favorite web page.
(Fun fact: B3's backend contains a machine description language that gets compiled to C++ code by a ruby script, opcode_generator.rb. We use Ruby a lot.)