定义一个用于显示验证码的canvas

<canvas width="100" height="40"></canvas>

生成JS的业务逻辑

    // 获取canvas
    let canvas = document.querySelector("canvas")
    let context = canvas.getContext("2d");
        
    // 定义初始化验证码内容
    let nums = ["1", "2", "3", "4", "5", "6", "7", "8", "9", "0", 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R','S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x','y', 'z'];

    drawCode()

    // 绘制验证码
    function drawCode() {
        context.beginPath()

        /* 绘制背景色 */
        context.fillStyle = "cornflowerblue";
        context.fillRect(0, 0, canvas.width, canvas.height)

        /* 绘制验证码 */
        context.fillStyle = "white";
        context.font = "25px Arial";

        let rand = [], x = [], y = []
        for (let i = 0; i < 5; i++) {
            rand[i] = nums[Math.floor(Math.random() * nums.length)]
            x[i] = i * 16 + 10;
            y[i] = Math.random() * 20 + 20;
            context.fillText(rand[i], x[i], y[i]);
        }
        
        /* rand就是生成后的结果， 后面用来判断验证码输入框是否与该值相等 */
        console.log(rand);

        //画3条随机线
        for (let i = 0; i < 3; i++) {
            drawline(canvas, context);
        }

        // 画30个随机点
        for (let i = 0; i < 30; i++) {
            drawDot(canvas, context);
        }
    }

    // 随机线
    function drawline(canvas, context) {
        //随机线的起点x坐标是画布x坐标0位置，y坐标是画布高度的随机数
        context.moveTo(Math.floor(Math.random() * canvas.width), Math.floor(Math.random() * canvas.height)); 
        //随机线的终点x坐标是画布宽度，y坐标是画布高度的随机数
        context.lineTo(Math.floor(Math.random() * canvas.width), Math.floor(Math.random() * canvas.height));
        context.lineWidth = 0.5;
        context.strokeStyle = 'rgba(50,50,50,0.3)';
        context.stroke();   
    }

    // 随机点
    function drawDot(canvas, context) {
        let px = Math.floor(Math.random() * canvas.width);
        let py = Math.floor(Math.random() * canvas.height);
        context.moveTo(px, py);
        context.lineTo(px + 1, py + 1);
        context.lineWidth = 0.2;
        context.stroke();
    }