<canvas id="myCanvas" width="200" height="100" style="border:1px solid #000000;"></canvas>

    var cavs = document.getElementById("cavs");           //获取canvas
    var ctx = cavs.getContext("2d");                     //可以理解为生成一个2d画笔
        ctx.moveTo(100, 100);                           //落笔点
        ctx.lineTo(200, 200);                          //结束点

         ctx.strokeStyle = "#f00"                 //线条的颜色2
         ctx.lineWidth = 10;                    //线条粗细

　　　　　 ctx.stroke();            //绘制线条

　　　　　ctx.fillStyle = "yellow";            //填充颜色
　　　　　ctx.fill();                //进行填充

         ctx.beginPath();    //开始
         ctx.closePath();    //结束

         var img = ctx.getImageData(x, y, width, height);     //截取canvas中图案
         ctx.putImageData(img, width, height)            //将img放到canvas中
         ctx.clearRect(x,y,canvas.width,canvas.height)         //清除画布

<div class="wrapper">
    <canvas id="cavs" width="1000" height="500"></canvas>
    <ul class="btn-list">
        <li><input type="color" id="colorBoard"></li>
        <li><input type="button" id="cleanBoard" value="清屏"></li>
        <li><input type="button" id="eraser" value="橡皮"></li>
        <li><input type="button" id="rescind" value="撤销"></li>
        <li><input type="range" id="lineRuler" value="线条" min="1" max="30"></li>
    </ul>
</div>

*{
  padding: 0;
  margin: 0;
}
.wrapper{
  margin: 15px;
}
.wrapper canvas{
  border:1px solid #000;
  border-radius: 25px;
  box-shadow:10px 10px 5px #999;
  margin-bottom: 20px;
}
.wrapper ul{
  width: 1000px;
  text-align: center;
}
.wrapper ul li{
  display: inline-block;
  margin-left: 40px;
}
.wrapper ul li input{
  display: block;
  background: #ddd;
  color: #000;
  border-radius: 25px;
  border:none;
  padding: 10px 20px;
  font-size: 15px;
  transition-duration: 0.2s;
}
.wrapper ul li input:hover{
  background: #999;
  border: 1px solid #f00;
  box-shadow: 0 12px 16px 0 rgba(0,0,0,0.3);
}

function ID(str) {
  return document.getElementById(str);
}
var darwingLineObj = {
  cavs:this.ID("canvas"),
  color:this.ID("color"),
  clear:this.ID("clear"),
  eraser:this.ID("eraser"),
  rescind:this.ID("rescind"),
  weight:this.ID("weight"),
  bool:false,
  arrImg:[],
  //初始化
  init:function(){
    this.draw();
  },
  draw:function(){ 
    var cavs = this.cavs,
        self = this,
        ctx = cavs.getContext("2d");
        ctx.lineWidth = 15;
        ctx.lineCap = "round";          //线条始终的样式
        ctx.lineJoin = "round";         //转弯的圆角
 
    var c_x = cavs.offsetLeft,          //元素距离左侧的位置
        c_y = cavs.offsetTop;           //canvas距离顶部
        
    cavs.onmousedown = function(e){
      e = e || window.event;
      self.bool = true;
      var m_x = e.pageX - c_x;
      var m_y = e.pageY - c_y;
      ctx.beginPath();
      ctx.moveTo(m_x,m_y);   //鼠标在画布上的触点
 
      var imgData = ctx.getImageData(0, 0, cavs.width, cavs.height);  //将每次画完拷贝到数组中
      self.arrImg.push(imgData);
    }
    cavs.onmousemove = function(e){
        if(self.bool){
          ctx.lineTo(e.pageX-c_x, e.pageY-c_y);
          ctx.stroke();
        }
      }
    cavs.onmouseup = function(e){
      self.bool = false;
      ctx.closePath();
    }
    self.color.onchange = function(e){      //设置颜色
      e = e || window.event;
      //console.log(e.target.value)
      ctx.strokeStyle = e.target.value;
    }
    self.clear.onclick = function(){
      ctx.clearRect(0,0,cavs.width,cavs.height)
    }
    self.eraser.onclick = function(){
      ctx.strokeStyle = "#fff";
    }
    self.rescind.onclick = function(){            //撤销的重点难点
      if (self.arrImg.length>0) {
        ctx.putImageData(self.arrImg.pop(),0,0)
      }
    }
    self.weight.onchange = function(e){       //设置线条粗细
      //console.log(e.target.value)
      ctx.lineWidth = e.target.value;     
    }
   }//draw end
}
darwingLineObj.init();