How to apply paragraph style ti lines read from a csv file.

Hi @mpp_piotrp, you are almost there! I've made some changes to your script so please have a careful look through and see what I've done differently. (Edit: fixed a bug I introduced and added a little more documentation.)

function main() {

    // read csv file
    var csv_file = File('~/Desktop/Test.csv');
    if (!csv_file.exists) {
        alert('CSV file doesn\'t exist');
        return;
    }
    csv_file.open('r');
    var contents = csv_file.read();
    csv_file.close();
    var rows = contents.split("\n");
    var dataRowCount = rows.length;

    // add contents to document
    var myDoc = app.activeDocument;
    var myTextFrame = myDoc.textFrames[0];
    var BOLD = myDoc.paragraphStyles.item("bold");
    var NORMAL = myDoc.paragraphStyles.item("regular");

    for (var i = 0; i < dataRowCount; i++) {

        var record = rows[i].split(",");

        if (
            myTextFrame.characters.length > 0
            && myTextFrame.characters[1].contents !== '\r'
        )
            myTextFrame.insertionPoints[-1].contents = '\r';

        // this is where you format the line
        var lineContents = record.join(' ')
            // remove double-spaces
            .replace(/\s+/g, ' ');

        myTextFrame.insertionPoints[-1].contents = lineContents;

        if (!myTextFrame.paragraphs[i].isValid)
            continue;

        if (record[10] == "bold")
            myTextFrame.paragraphs[i].appliedParagraphStyle = BOLD;
        else
            myTextFrame.paragraphs[i].appliedParagraphStyle = NORMAL;

    }

}

app.doScript(main, ScriptLanguage.JAVASCRIPT, undefined, UndoModes.ENTIRE_SCRIPT, 'Enter CSV');

Some of the problems were due to incorrect capitalisation (eg. MyDoc vs myDoc are two different objects). I also put a test to check if the csv file exists. I also used InsertionPoints rather than replacing the whole contents (because replacing the whole textframe's contents will apply the first character's paragraph style to the whole text). I also changed some of your variable names to distinguish better between objects, eg. data lines I called rows so they can't be confused with indesign paragraph lines. That is up to you. I think it is a good practice to apply a paragraph style to every paragraph, not just the "bold" ones, so I made a style called "regular", too.

Well I hope that gives you a boost to take it to the next level. You are doing really well so far.

- Mark