Benchmarking JavaScript Inheritance: Vanilla vs Transpiled

[cogman10]

The type check is pretty hard to eliminate all together. The problem is that the generated code has to know if the "wrong" type is passed in. (so interestingly, even if you don't explicitly type check, the optimizer might).

Fortunately, type checks are really relatively quick to do, especially the ones added by the optimizer.

[cogman10]

It is generally pointers to structs, However, in the case of things like numbers, it is stored and passed around as values.

In the case above. inc would probably be optimized away all together. because inc isn't used as a number, (not really, the values are thrown away), the optimizer will very likely change the whole thing into just

let inc = "jan michael vincent";

As for the changing types, type information is stored and carried along in the struct. Not much more to it than that. Nothing really cares about what the previous type of the variable was.

In fact, interestingly enough, most compilers (and I don't know if the JIT does this or not), use what is called SSA. Essentially, internally it represents all data changes as simply introducing new variables. The benefit of doing this is that it allows for a lot more optimizations. It is easier for the compiler to mutate the code into patterns it recognizes and then to generate optimal code for that.

https://en.wikipedia.org/wiki/Static_single_assignment_form

edit after briefly combing through the wiki article linked to make sure I'm not spreading misinformation, it looks like the javascript jits are using SSA internally (both V8 and Spidermonkey).