What is the JavaScript string newline character?

I've just tested a few browsers using this silly bit of JavaScript:

function log_newline(msg, test_value) {
  if (!test_value) { 
    test_value = document.getElementById('test').value;
  }
  console.log(msg + ': ' + (test_value.match(/\r/) ? 'CR' : '')
              + ' ' + (test_value.match(/\n/) ? 'LF' : ''));
}

log_newline('HTML source');
log_newline('JS string', "foo\nbar");
log_newline('JS template literal', `bar
baz`);

<textarea id="test" name="test">

</textarea>

IE8 and Opera 9 on Windows use \r\n. All the other browsers I tested (Safari 4 and Firefox 3.5 on Windows, and Firefox 3.0 on Linux) use \n. They can all handle \n just fine when setting the value, though IE and Opera will convert that back to \r\n again internally. There's a SitePoint article with some more details called Line endings in Javascript.

Note also that this is independent of the actual line endings in the HTML file itself (both \n and \r\n give the same results).

When submitting a form, all browsers canonicalize newlines to %0D%0A in URL encoding. To see that, load e.g. data:text/html,<form><textarea name="foo">foo%0abar</textarea><input type="submit"></form> and press the submit button. (Some browsers block the load of the submitted page, but you can see the URL-encoded form values in the console.)

I don't think you really need to do much of any determining, though. If you just want to split the text on newlines, you could do something like this:

lines = foo.value.split(/\r\n|\r|\n/g);

Yes, it is universal.

Although '\n' is the universal newline characters, you have to keep in mind that, depending on your input, new line characters might be preceded by carriage return characters ('\r').

Yes, use \n, unless you are generating HTML code, in which case you want to use <br />.

printAccountSummary: function()
    {return "Welcome!" + "\n" + "Your balance is currently $1000 and your interest rate is 1%."}
};
console.log(savingsAccount.printAccountSummary()); // Method

Prints:

Welcome!
Your balance is currently $1000 and your interest rate is 1%.

I had the problem of expressing newline with \n or \r\n.
Magically the character \r which is used for carriage return worked for me like a newline.
So in some cases, it is useful to consider \r too.

I believe it is -- when you are working with JavaScript strings.

If you are generating HTML, though, you will have to use <br /> tags (not \n, as you're not dealing with JavaScript any more).

A practical observation... In my Node.js script I have the following function:

function writeToLogFile (message) {
    fs.appendFile('myserverlog.txt', Date() + " " + message + "\r\n", function (err) {
        if (err) 
            throw err;
    });
}

First, I had only "\n", but I noticed that when I open the log file in Notepad, it shows all entries on the same line. Notepad++, on the other hand, shows the entries each on their own line. After changing the code to "\r\n", even Notepad shows every entry on its own line.