JSFiddle - React, Tailwind, and code Playground

HTML

<div id="container">
  <div id="lines">
  "Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum."
  </div>  
</div>

CSS

#container {
  line-height: 2em;
}

#lines {
  box-sizing: border-box;
  padding: 2em;
}

JavaScript

function calculateLineCount(element) {
  var lineHeightBefore = element.css("line-height"),
      boxSizing        = element.css("box-sizing"),
      height,
      lineCount;

  // Force the line height to a known value
  element.css("line-height", "1px");

  // Take a snapshot of the height
  height = parseFloat(element.css("height"));

  // Reset the line height
  element.css("line-height", lineHeightBefore);

  if (boxSizing == "border-box") {
    // With "border-box", padding cuts into the content, so we have to subtract
    // it out
    var paddingTop    = parseFloat(element.css("padding-top")),
        paddingBottom = parseFloat(element.css("padding-bottom"));
        
    height -= (paddingTop + paddingBottom);
  }

  // The height is the line count
  lineCount = height;

  return lineCount;
}

alert(calculateLineCount($('#lines')));