Class Range

// Bold all selected text.
var selection = DocumentApp.getActiveDocument().getSelection();
if (selection) {
  var elements = selection.getRangeElements();
  for (var i = 0; i < elements.length; i++) {
    var element = elements[i];

    // Only modify elements that can be edited as text; skip images and other non-text elements.
    if (element.getElement().editAsText) {
      var text = element.getElement().editAsText();

      // Bold the selected part of the element, or the full element if it's completely selected.
      if (element.isPartial()) {
        text.setBold(element.getStartOffset(), element.getEndOffsetInclusive(), true);
      } else {
        text.setBold(true);
      }
    }
  }
}