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Comment by YorickPeterse | Hacker News Reader

Ruby's meta-programming capabilities is why optimising Ruby is so darn difficult. It's extremely powerful, but also complicates a lot of things.

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I reason the difference is mostly when metaprogramming can happen. In Java, redefining or adding code is very explicit, it can't just happen whenever. Same with C#. And there are lots of rules. A class can't modify itself, and there's limits to what changes you can make. And to make changes you must have control over the "classloader" that loaded that code.

JS on the other hand can do most of what ruby does, to my knowledge. Objects are key value pairs so you're free to mess with them in virtually any way you please. You can also mess with the inheritance by altering JS prototype chains.

I don't think metaprogramming itself has much to do with the speed of Ruby, with my admittedly limited knowledge of this stuff

However, JS does not treat everything as an object as pervasively as Ruby does. A JS object and a JS integer are two different abstract data types. A Ruby number literal "1" is treated as an object of the Integer class. There are no separate abstract data type other than an object. All operators for an Integer can be overriden (in runtime), or perhaps, a specific object's methods can be overridden. Now granted, the runtime cheats and implements certain things in C ... but those can be overriden during runtime ...

If you want to find out more about the limits of what can be done to optimize Ruby, check out the Truffle project. That came out of someone's PhD dissertation on novel methods for doing JIT optimization for Ruby. It is sufficiently difficult and novel to warrant awarding a PhD for. Last time I heard, Truffle still could not run Rails.

A JS integer is actually an instance of the "Number" object, you can to a small degree alter fundamental behavior even with primitive types

I stand corrected.

No it is not. It is a primitive, this difference is well defined in the ECMAScript spec. Same for strings. An instanceof String is strictly not the same as a string primitive (and there are runtime consequences).

You need to enclose numbers to access the prototype: 1.toString is undefined, whereas (1).toString() is "1"

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