get-line-number-by-char-index.js

// Find which line number a 0-based index falls on by splitting a line
// of text delimited by '\n'

function getLineNumberByCharIndex(index) {
  var TEXT = 'abc\ndef\nghijkl\nmnop';
  var line = 0;
  var i;

  for (i = 0; i <= index; i++) {
    if (TEXT[i] === '\n') line++;
  }

  return line;
}

// => 3, 0, 1, 2
console.log(
  getLineNumberByCharIndex(14),
  getLineNumberByCharIndex(0),
  getLineNumberByCharIndex(4),
  getLineNumberByCharIndex(8)
);