如何知道输入框中文本的哪一部分发生了更改?

$('#input').on('input', onChange);

let before = '';

function onChange(target){
  const { value: after } = target.currentTarget;
  // Length of change, negative if removed
  const lengthChange = after.length - before.length;
  let changed;
  // If added some, get the new stuff
  if (lengthChange > 0) {
    changed = after.slice(after.length - lengthChange);
  // Other wise get the old stuff
  } else {
    changed = "-" + before.slice(before.length + lengthChange);
  }
  console.log(`before: ${before}, change: ${changed}, after: ${after}`);
  before = after;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<input id="input" type="text" />

$('#input').on('input', onChange);

let before = '';

function onChange(target){
    const { selectionStart: start, value: after } = target.currentTarget;
    
    // calculate the different starting points from both direction
    let p1 = 0, p2 = 0;
    // calclate the end point first
    for(; p2 < after.length - start; p2++){
        if(before[before.length - 1 - p2] === undefined || before[before.length - 1 - p2] !== after[after.length - 1 - p2]){
            break;
        }
    }
    // calclate start point
    for(; p1 < after.length - p2 - (after.length > before.length ? 1 : 0); p1++){
        if(before[p1] !== after[p1] || p1 + p2 >= before.length){
            break;
        }
    }
    
    let insertedText = '';
    let insertPosition = -1;
    
    let deletedTextCount = 0;
    let deletePosition = -1;
    
    // has delete
    if(p1 + p2 < before.length){
        deletedTextCount = before.length - (p1 + p2);
        deletePosition = p1;
    }

    // has add
    if(p1 + p2 < after.length){
        insertedText = after.substr(p1, after.length - (p1 + p2));
        insertPosition = p1;
    }
    
    console.log(`deletePosition: ${deletePosition}, deletedTextCount: ${deletedTextCount}, insertPosition: ${insertPosition}, insertedText: "${insertedText}"`);
    
    before = after;
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<input id="input" type="text" />