React中如何实现循环创建div并添加行号？

import react from "react";
import reactdom from "react-dom";
import "./styles.css";

function app() {
  const itemdata = new array(500).fill(0);
  const indexdata = new array(20).fill(0);

  return (
    <div classname="app">
      <div classname="left-box">
        {indexdata.map((item, index) => (
          <div
            key={index}
            style={{
              fontsize: "9px",
              width: "19px",
              height: "20px",
              margin: "2px",
            }}
          >
            {index}
          </div>
        ))}
      </div>
      <div classname="right-box">
        {itemdata.map((item, index) => (
          <div
            key={index}
            style={{
              display: "inline-block",
              width: "9px",
              height: "9px",
              margin: "2px",
              backgroundcolor: item.selected ? "green" : "lightgreen",
            }}
          ></div>
        ))}
      </div>
    </div>
  );
}

const rootelement = document.getelementbyid("root");
reactdom.render(<app />, rootelement);

.App {
  font-family: sans-serif;
  text-align: center;
  overflow: hidden;
  position: relative;
}

.left-box {
  position: absolute;
  left: 0;
  top: 0;

  width: 19px;
  overflow: hidden;
}

.right-box {
  margin-left: 19px;
}