js怎么做验证码

<code class="javascript">const canvas = document.createElement("canvas");
canvas.width = 100;
canvas.height = 50;
const ctx = canvas.getContext("2d");</code>

<code class="javascript">function generateString(length) {
  const chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";
  let result = "";
  for (let i = 0; i < length; i++) {
    result += chars.charAt(Math.floor(Math.random() * chars.length));
  }
  return result;
}</code>

<code class="javascript">const text = generateString(6);
ctx.font = "bold 16px Arial";
ctx.fillText(text, 10, 25);</code>

<code class="javascript">// 添加杂色
for (let i = 0; i < 100; i++) {
  ctx.fillStyle = "rgba(0, 0, 0, 0.1)";
  ctx.fillRect(Math.random() * 100, Math.random() * 50, 1, 1);
}

// 扭曲文本
for (let i = 0; i < 20; i++) {
  ctx.translate(Math.random() * 2, Math.random() * 2);
  ctx.rotate(Math.random() * Math.PI / 180);
}</code>

<code class="javascript">const captcha = canvas.toDataURL("image/png");</code>