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js实现图片验证码

2026-04-03 19:37:26JavaScript

实现图片验证码的基本思路

图片验证码通常由随机生成的字符或数字组成，经过扭曲、干扰线、噪点等处理后呈现为图像。以下是一个基于JavaScript和Canvas的实现方法：

生成随机字符串

function generateRandomString(length) {
  const chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
  let result = '';
  for (let i = 0; i < length; i++) {
    result += chars.charAt(Math.floor(Math.random() * chars.length));
  }
  return result;
}

创建Canvas绘制验证码

function createCaptcha() {
  const canvas = document.getElementById('captchaCanvas');
  const ctx = canvas.getContext('2d');
  const captchaText = generateRandomString(6);

  // 清除画布
  ctx.clearRect(0, 0, canvas.width, canvas.height);

  // 设置背景色
  ctx.fillStyle = '#f5f5f5';
  ctx.fillRect(0, 0, canvas.width, canvas.height);

  // 绘制文字
  ctx.font = '30px Arial';
  ctx.fillStyle = '#333';
  ctx.textAlign = 'center';

  // 添加文字扭曲效果
  for (let i = 0; i < captchaText.length; i++) {
    ctx.save();
    ctx.translate(30 + i * 20, 30);
    ctx.rotate((Math.random() - 0.5) * 0.4);
    ctx.fillText(captchaText[i], 0, 0);
    ctx.restore();
  }

  // 添加干扰线
  for (let i = 0; i < 5; i++) {
    ctx.strokeStyle = getRandomColor();
    ctx.beginPath();
    ctx.moveTo(Math.random() * canvas.width, Math.random() * canvas.height);
    ctx.lineTo(Math.random() * canvas.width, Math.random() * canvas.height);
    ctx.stroke();
  }

  // 添加噪点
  for (let i = 0; i < 100; i++) {
    ctx.fillStyle = getRandomColor();
    ctx.fillRect(
      Math.random() * canvas.width,
      Math.random() * canvas.height,
      1,
      1
    );
  }

  return captchaText;
}

function getRandomColor() {
  const letters = '0123456789ABCDEF';
  let color = '#';
  for (let i = 0; i < 6; i++) {
    color += letters[Math.floor(Math.random() * 16)];
  }
  return color;
}

HTML结构

<div class="captcha-container">
  <canvas id="captchaCanvas" width="200" height="60"></canvas>
  <button id="refreshCaptcha">刷新</button>
</div>
<input type="text" id="captchaInput" placeholder="输入验证码">
<button id="verifyCaptcha">验证</button>

验证逻辑

let correctCaptcha = '';

document.addEventListener('DOMContentLoaded', function() {
  correctCaptcha = createCaptcha();

  document.getElementById('refreshCaptcha').addEventListener('click', function() {
    correctCaptcha = createCaptcha();
  });

  document.getElementById('verifyCaptcha').addEventListener('click', function() {
    const userInput = document.getElementById('captchaInput').value;
    if (userInput === correctCaptcha) {
      alert('验证码正确');
    } else {
      alert('验证码错误');
      correctCaptcha = createCaptcha();
    }
  });
});