Open new document in landscape orientation

Nothing wrong with Charu's reply. Here's a simpler version that doesn't worry about the exact position of the artboard since you're creating a brand new blank document. If you wanted to convert an existing artboard with artwork on it to landscape, you'd want to have some logic to make sure the centerpoint (or any other reference point you choose) stays consistent.

function NewDocLandscapeArtboard()
{
	const doc = app.documents.add();
	const ab = doc.artboards[0];
	var rect = ab.artboardRect;
	const w = rect[2]-rect[0];
	const h = rect[1] - rect[3];
	const cp = [rect[0] + w/2,rect[1] - h/2];

	if(w < h)
	{
		ab.artboardRect = [rect[0],rect[1],rect[0] + h,rect[1] - w];
	}
}
NewDocLandscapeArtboard();

Hi,

I am sure sure if there is a firect way to do this.

You can rotate the artboards after adding the new document if heightwidth is less than to have landscape view. Sample code below and references

var doc = app.documents.add()
if (doc.width < doc.height) {
    var currentArtboard = doc.artboards.getActiveArtboardIndex();
    var currentArtboardRect = doc.artboards[currentArtboard].artboardRect; // Rect, which is an array;
    var currentLeft = currentArtboardRect[0];
    var currentTop = currentArtboardRect[1];
    var currentRight = currentArtboardRect[2];
    var currentBottom = currentArtboardRect[3];
    var currentWidth = Math.abs(currentRight - currentLeft);
    var currentHeight = Math.abs(currentBottom - currentTop);

    var centerX = currentLeft + (currentWidth / 2);
    var centerY = currentTop - (currentHeight / 2); // Y axis is inverted

    var newArtboardRect = [];
    var newWidth = currentHeight;
    var newHeight = currentWidth;

    // Rotate the artboard 90 degrees

    newArtboardRect[0] = centerX - (newWidth / 2);
    newArtboardRect[1] = centerY + (newHeight / 2); // Y axis is inverted
    newArtboardRect[2] = centerX + (newWidth / 2);
    newArtboardRect[3] = centerY - (newHeight / 2); // Y axis is inverted

    doc.artboards[currentArtboard].artboardRect = newArtboardRect;
}

https://github.com/mprewitt/AI-Rotate-Artboard

If I found a direct way, will post here.