according to unity document (1) If the string that has been written to memory using stringToUTF8 is a return value, then the il2cpp runtime will take care of freeing the memory. In the case of passing the string (e.g. using dynCall method) how I should get rid of the written string on memory later on.
here is the related part of my jslib plugin code:
var str = 'something';
var bufferSize = lengthBytesUTF8(str) + 1;
var buffer = _malloc(bufferSize);
stringToUTF8(str, buffer, bufferSize);
Runtime.dynCall('vi', CallbackMethod, [buffer]);