I have done it 😁 it need some polish but here is the code to accomplish it using the %SyntaxColor library:

Method ExtractJsFunctionBody(jsCodePath As%String, fnName As%String)
{
  
    #; Reading from a file, writing to a temporary streamset syn = ##class(%SyntaxColor).%New(), in = ##class(%Stream.FileCharacter).%New(), out = ##class(%Stream.TmpCharacter).%New()
    // set js content in content var    set content = in.Read($$$MaxStringLength)
   do InStream.Rewind()
   #; Need the "K" flag to get JSON outputdo in.LinkToFile(jsCodePath), syn.Color(in,out,"JS","K")
   #; Get a %DynamicArray from the streamset lines = ##class(%DynamicArray).%FromJSON(out)
   // get list by line of jsCodeset lineList = $listfromstring(content, $char(10))

   // get pos and line of function def begining set iterLines = lines.%GetIterator()
   while iterLines.%GetNext(.linenumber , .line, .type) {

      set value = line.%Get(1)
      if ((value.%Get("s") = 16) &&  (value.%Get("c") = 8)){  
         set value = line.%Get(3)
         if (value.%Get("s") = 14 ){
            set tempFnName = $extract($list(lineList, linenumber + 1),value.%Get("p") + 1, value.%Get("p") + value.%Get("c"))
            if (tempFnName = fnName ){
               set functionLineBegin = linenumber + 1quit
            }
         }
      }
   }
   
   // get pos of function def endingset functionLineEnd = ..braketCounter(iterLines, lineList)
   
   //extract functiun Bodyset functionBody = $listtostring($list(lineList,functionLineBegin,functionLineEnd),$char(10))
   write !!!, functionBody
}

/// Count braket and return the line of the corresponding ending one take an iterrator at the line of the begining of the search and a list of the lines
Method braketCounter(iterLines As%Iterator.AbstractIterator, lineList As%DynamicArray) As%Integer
{
   set countBraket = 1while iterLines.%GetNext(.linenumber , .line, .type) { // we keep the same iterator to continue from where we whereset iterTokens = line.%GetIterator()
      while iterTokens.%GetNext(.key , .value, .type){
         set token = $extract($list(lineList, linenumber + 1),value.%Get("p") + 1, value.%Get("p") + value.%Get("c") )
         if (value.%Get("s") = 4) {
            if (  token = "{"){
               set countBraket = countBraket + 1}
            elseif ( token = "}"){
               set countBraket = countBraket - 1}
            if (countBraket = 0){
               return linenumber 
            }
         }
         
      }
   }
   return"error"
}