Thread: What does it mean when people say that Python and JS Libraries are "written in C"?

I've heard this said before and it half makes intuitive sense. One example is this video at the 1 minute mark: Why C Programming Is Awesome - YouTube

I think it might have something to do with encapsulation (information hiding) and/or compiling to byte code, which is what all languages are at the end of the day.

Is it that when you execute a JavaScript program, it actually calls down to C function's to accomplish some of the tasks?

I was hoping someone could explain this.

Also of tangential note: is there any way to look "under the hood" of JavaScript or Python programs to see the C code that is involved? All the C will probably have been compiled, but are there any resources that show the source code?

Not C necessarily, but yeah, interpreted languages such as JS and Py use... well... an interpreter. That interpreter needs to be as efficient as possible, so it's only logical that efficient, close-to-the-metal languages such as C or C++ would be used for their implementation.

For example, here's Spider Monkey, a JS engine for firefox.
SpiderMonkey - Mozilla | MDN

To answer your main question:
"Calling down to C to accomplish some tasks" refers to when interpreted languages call functions which are implemented in C instead of that language. So in Python, the interpreter is made to know that when it sees those particular functions, to look at the attached compiled binaries instead of the python source. In javascript it's similar except the C functions must be part of the browser, so you can't just attach any c code. Technically the function aren't necessarily written in C, but they must be made to look like they were by having "C linkage".

It's also true that interpreters are themselves written in native languages like C. This is technically separate from "calling down to c", except that standard library functions may call down to C, and are effectively part of the interpreter. Unlike calling down to c, interpreters themselves do not need C linkage, and so are more easily written in other languages.