How is the input value of the V8::String::AsciiValue method handled?

Question

JavaScript

Hello dear nodejs programmers.
Please tell me how this method works... V8::String::AsciiValue
what needs to be passed as input and what we get as output... and how the encoding of the input text data affects the result.
p.s. I'm not a nodejs, javascript, or even a c/c++ programmer. I work only with php, but at work I had to dig into one module for nodejs (nodem module) in the sources of which (c / c ++) there was this method that causes a problem with non-UTF-8 data encoding ... (the module works with UTF-8 ok, different kernel method V8)