Function js_string_to_code_units 

pub fn js_string_to_code_units(
    cx: &mut JSContext,
    data: HandleValue<'_>,
    target: MutableHandleString<'_>,
) -> Result<ConversionResult, Error>

https://webidl.spec.whatwg.org/#js-DOMString Implements the js to DOMstring conversion, but using an ad-hoc data structure, because current DOMString implementation does not preserve the exact sequence of code units.