Non-ECMAscript code for function

Question

I'm reading through the ECMA Script 2015 specification.

Under functions I see:

In addition to its properties, a function contains executable code and state that determine how it behaves when invoked. A function’s code may or may not be written in ECMAScript (emphasis added).

Under what circumstances would a function's code not be written in ECMA script?

Answer 1

The native functions provided by the execution environment (like the String and Array classes, or setTimeout, or the browser DOM) are frequently written (or powered by other functions written) in C.

Answer 2

Under what circumstances would a function's code not be written in ECMA script?

Core functions provided by the JavaScript engine. E.g. Array.prototype.find:

> Array.prototype.find
find() { [native code] }

For example V8, Chrome's JavaScript engine, is implement in C++, so that method is implemented in C++.