Normalize line endings in a JavaScript string

Historically, different operating systems have used different characters to represent line endings in text files. For example, Windows uses '\r\n' (carriage return + line feed), Unix-like systems use '\n' (line feed), and older Mac systems used '\r' (carriage return). All of these differences can cause issues when working with text files or strings.

Normalization is the process of converting all line endings in a string to a consistent format. Often, you'll want to convert all line endings to a specific format, such as '\r\n' or '\n'.

Luckily, all you need to do is use String.prototype.replace() with a regular expression to match and replace line endings. As the sequences of characters are known and ordered consistently, you need only check for an optional carriage return (\r?) followed by a line feed (\n).

const normalizeLineEndings = (str, normalized = '\n') =>
  str.replace(/\r?\n/g, normalized);

normalizeLineEndings('This\r\nis a\nmultiline\nstring.\r\n');
// 'This\nis a\nmultiline\nstring.\n'

normalizeLineEndings('This\r\nis a\nmultiline\nstring.\r\n', '\r\n');
// 'This\r\nis a\r\nmultiline\r\nstring.\r\n'