SOURCE

var cas=document.querySelector('#canvas');
var ctx=cas.getContext('2d');     //构建绘图环境
function randomCode(position,needDisturbPoint,needDisturbLine){
    //position Object x:起始横坐标 y:起始纵坐标 w:矩形宽 h:矩形高
    function randNum(min,max){
        //获取随机数
        return Math.floor(Math.random()*(max-min+1)+min);
    }
    function randColor(min,max){
        //获取一个随机颜色
        var r = randNum(min,max);
        var g = randNum(min,max);
        var b = randNum(min,max);
        return 'rgb('+r+','+g+','+b+')';
    }
    function drawRect(x,y,w,h){
        //绘制一个带颜色的矩形
        ctx.fillStyle=randColor(170,250);   //这里背景色的取值范围为[170,250],颜色比较适中
        ctx.fillRect(x,y,w,h);    //绘制矩形
    }
    function get4RandomCode(){
        var code = "";
        var data='qwertyuiopasdfghjklzxcvbnmQWERTYUIOPASDFGHJKLZXCVBNM1234567890';
        for(var i=0;i<120;i+=30){　　//循环四次,取四个字
            var c=data[randNum(0,data.length-1)]; //index为随机数,[0,data.length-1]  取字(随机)
            code = code + c;
            ctx.fillStyle=randColor(60,160);    //字体颜色
            ctx.font=randNum(15,40)+'px SimHei';  //字体大小,字体
            ctx.fillText(c,i+randNum(5,12),randNum(15,30));   //字体填充(字内容,x轴,y轴)位置都是随机
        }
        return code.toUpperCase(); 
    }
    function getDisturbLine(){
        for (var i=0;i<10;i++) {
            ctx.beginPath(); //路径开始,子路经的集合
            ctx.moveTo(randNum(0,120),randNum(0,120));//起始点(x,y),都随机
            ctx.lineTo(randNum(0,120),randNum(0,120));//终止点(x,y)
            ctx.strokeStyle=randColor(60,160);//路径的颜色
            ctx.stroke();  //将上面的两个点连接起来
        }
    }
    function getDisturbPoint(){
        ctx.beginPath(); //路径开始
　　    ctx.arc(randNum(0,120),randNum(0,30),randNum(1,5),0,2*Math.PI);//画圆(x,y,z,0,2*Math.PI) x坐标,y坐标,半径,完整圆
　　    ctx.strokeStyle=randColor(60,160);
　　    ctx.stroke();
        ctx.fill();
    }
    drawRect(position.x,position.y,position.w,position.h);
    var code = get4RandomCode();
    randColor(0,255);
    if(needDisturbLine) getDisturbLine();
    if(needDisturbPoint) getDisturbPoint();
    return code;
}

let code = randomCode({x:0,y:0,w:120,h:30},true,true);
console.log(code);

<canvas id="canvas" width="120" height="30" style="cursor: pointer;"></canvas>