Splitting a string into multiple lines in javascript

Very straightforward answer to your question:

function customSplit(str, maxLength){
    if(str.length <= maxLength)
        return str;
    var reg = new RegExp(".{1," + maxLength + "}","g");
    var parts = str.match(reg);
    return parts.join('\n');
}

You need a function like the following:

function split(str, maxWidth) {
  const newLineStr = "\n";
  done = false;
  res = '';
  do {
    found = false;
    // Inserts new line at first whitespace of the line
    for (i = maxWidth - 1; i >= 0; i--) {
      if (testWhite(str.charAt(i))) {
        res = res + [str.slice(0, i), newLineStr].join('');
        str = str.slice(i + 1);
        found = true;
        break;
      }
    }
    // Inserts new line at maxWidth position, the word is too long to wrap
    if (!found) {
      res += [str.slice(0, maxWidth), newLineStr].join('');
      str = str.slice(maxWidth);
    }

    if (str.length < maxWidth)
      done = true;
  } while (!done);

  return res + str;
}

function testWhite(x) {
  const white = new RegExp(/^\s$/);
  return white.test(x.charAt(0));
};
console.log(split("sometext", 5));
console.log(split("Hello", 2));

What kind of interface are you building? If it's a web interface, you should style the string on the front end, instead of modifying it on the data layer.

If it's a text based interface and you really need to do this, then you can get the first n characters while there is a non-empty string, then join with '\n'. Assuming you have underscore:

function split(str, n) {
  let numberOfSegments = Math.ceil(_.size(str) / n);
  let segments = _.map(_.range(numberOfSegments), 
                       segmentIndex => str.substr(segmentIndex * n, n));
  return segments.join('\n');
}