test.ts

const avgSize = 20;
const scrollOffset = 225;

function measureRangeSize({ start, end }) {
  const size = 15;
  return (start + end) * size;
}

function getOffset(range, index) {
  const isInView = index > range.start && index < range.end;
  const direction = index < range.start ? 'before' : 'after';
  const start = direction === 'before' ? index : range.start;
  const end = isInView ? index : range.end;
  const delta = direction === 'before' ? index - start : index - end;

  let offset = 0;
  const renderedSize = measureRangeSize({
    start,
    end
  });

  if (!isInView) {
    if (direction === 'before') {
      offset = renderedSize - scrollOffset;
    } else {
      offset += offset + (delta * avgSize);
    }
  }
  
  return offset;
}

// const res = getOffset({ start: 0, end: 15 }, 5);
// console.log('In range:', res === 75);

const res2 = getOffset({ start: 15, end: 20 }, 5);
console.log('In range:', res2);