google-nextcloud-ical-convert/convert.ts

const decoder = new TextDecoder("utf-8");
const data = await Deno.readFile("ical.ics");
const text = decoder.decode(data);

function chunk(str, size) {
  const numChunks = Math.ceil(str.length / size)
  const chunks = new Array(numChunks)

  for (let i = 0, o = 0; i < numChunks; ++i, o += size) {
    chunks[i] = str.substr(o, size)
  }

  return chunks
}

const lines = text
	.split('\r\n')
	.reduce((newics, line) => {
		if (line.startsWith(' ')) {
			return newics + line.replace(/^ /, '');
		}

		return newics + '\n' + line;
	}, "")
	.split('\n')
	.map(line => line
		.replace(/<br>/g, '\\n')
		.replace(/<[^>]*>/g, '')
		.replace(/&gt\\;/g, '>')
		.replace(/&lt\\;/g, '<')
		.replace(/&nbsp\\;/g, ' ')
		.replace(/&amp\\;/g, '&')
		.replace(/&quot\\;/g, '"')
	)
	.reduce((allLines, line) => [
		...allLines,
		chunk(line, 40).join('\r\n '),
	], [])
	.join('\r\n');

console.log(lines);