Change textFrame content with given styled text

Let's say I have a given TextFrame, and I want to completely substitute its content with my own text - which is going to contain some text in bold and a newline - how should I go about it?

The final text should be something like:

<b>WORD</b><br>
NEW WORD

What I've tried so far:

var frame = app.activeDocument.textFrames.item("scr-hello");

var myStringsAndStyles = [
    {
      contents: "WORD",
      characterStyle: bold
    },
    {
      contents: "NEW WORD",
      characterStyle: none
    }
  ];

  with (frame.insertionPoints ) {
    for ( var i = 0, arrlength = myStringsAndStyles.length; i < arrlength; i++ ) {
      lastItem().properties = {
        contents: myStringsAndStyles[i].contents,
        appliedCharacterStyle: myStringsAndStyles[i].characterStyle
      }
    }
  }

The problem is:

• We are not replacing the TextFrame's content; we are adding to it
• The newline is not there
• The result is a mess (it's not even the two strings I supposedly added, one after the other)

It can be done this way:

var frame = app.activeDocument.textFrames.item('scr-hello');
frame.contents = 'WORD\nNEW WORD';              // fill the frame with the text
var line = frame.parentStory.texts[0].lines[0]; // get the first line from the frame
line.fontStyle = 'Bold';                        // assign Bold style

But you have to make sure that the font has Bold style. Some fonts have no additional styles.

Just in case, here is the extended example with character styles:

var doc = app.activeDocument;

// get the styles
var style_none = doc.characterStyles[0]; // '[None]'
var style_bold = doc.characterStyles.item('Bold');

// create the 'Bold' style if there is no such style
if (!style_bold.isValid) {
    style_bold = doc.characterStyles.add();
    style_bold.name = 'Bold';
    style_bold.fontStyle = 'Bold';
}

// get the frame and change its contents
var frame = app.activeDocument.textFrames.item('scr-hello');
frame.contents = 'WORD\nNEW WORD';
var story = frame.parentStory;

// remove the old formatting
story.texts.everyItem().appliedCharacterStyle = style_none;
story.texts.everyItem().clearOverrides();

// apply the 'Bold' style to the first line
var line = story.texts[0].lines[0].appliedCharacterStyle = style_bold;