How to insert text into specific line in textarea with jQuery?

We create a function that takes your text and a line number, splits the current text within the box by the new line character, add in the new text, and update the textarea box.

function addLine(text, line) {
  let ta = document.querySelector("textarea"), 
  split_text = ta.textContent.split(/\n/gmi);

    if (line <= split_text.length - 1) split_text.splice(line - 1, 0, text);
    else split_text.push(text);
  ta.textContent = split_text.join("\n");
}

addLine("This is my new text here!", 3);
addLine("This is another example!", 7);
addLine("And one more!", 2);

function addLine(text, line) {
let ta = document.querySelector("textarea"), 
split_text = ta.textContent.split(/\n/gmi);

  if (line <= split_text.length - 1) split_text.splice(line - 1, 0, text);
  else split_text.push(text);
  ta.textContent = split_text.join("\n");
}

addLine("This is my new text here!", 3);
addLine("This is another example!", 7);
addLine("And one more!", 2);

textarea {
  width: 300px;
  height: 300px;
}

<textarea>
line 1 text
line 2 text
line 3 text
line 4 text
line 5 text</textarea>