Detecting type of line breaks

Thank @Sam-Graham. I tried to produce an optimized way. Also, the output of the function is directly usable (see below example):

function getLineBreakChar(string) {
    const indexOfLF = string.indexOf('\n', 1);  // No need to check first-character

    if (indexOfLF === -1) {
        if (string.indexOf('\r') !== -1) return '\r';

        return '\n';
    }

    if (string[indexOfLF - 1] === '\r') return '\r\n';

    return '\n';
}

^{Note1: Supposed string is healthy (only contains one type of line-breaks).}

^{Note2: Supposed you want LF to be default encoding (when no line-break found).}

---

Usage example:

fs.writeFileSync(filePath,
        string.substring(0, a) +
        getLineBreakChar(string) +
        string.substring(b)
);

---

A utility (may be helpful):

function getLineBreakName(lineBreakChar) {
    return lineBreakChar === '\n' ? 'LF' : lineBreakChar === '\r' ? 'CR' : 'CRLF';
}

You would want to look first for an LF. like source.indexOf('\n') and then see if the character behind it is a CR like source[source.indexOf('\n')-1] === '\r'. This way, you just find the first example of a newline and match to it. In summary,

function whichLineEnding(source) {
     var temp = source.indexOf('\n');
     if (source[temp - 1] === '\r')
         return 'CRLF'
     return 'LF'
}

There are two popularish examples of libraries doing this in the npm modules: node-newline and crlf-helper The first does a split on the entire string which is very inefficient in your case. The second uses a regex which in your case would not be quick enough.

However, from your edit, if you want to determine which is more plentiful. Then I would use the code from node-newline as it does handle that case.