几行代码网页变成画布

paint,js

document.body.innerHTML = "<canvas id='cvs' width='1200px' height='600px'></canvas>";
cvs = document.getElementById('cvs');
ctx = cvs.getContext('2d');
cvs.onmousedown = function(){
  cvs.onmousemove = function(e){ctx.fillRect(e.offsetX,e.offsetY,2,2);console.log(e.clientX);}
};
cvs.onmouseup = function(){ cvs.onmousemove =function(e){}};

paint.js

document.body.innerHTML = "<canvas id='cvs' width='1200px' height='600px'></canvas>";
x = 0;
y = 0;
cvs = document.getElementById('cvs');
ctx = cvs.getContext('2d');
cvs.onmousedown = function(e){
  cvs.onmousemove = function(e){
    m = x + y;
    x = e.offsetX;
    y = e.offsetY;
    if (m > 0){
      ctx.lineTo(x,y);
      ctx.stroke();
    } else {ctx.moveTo(x, y);};
    ctx.moveTo(x, y);
  };
};
cvs.onmouseup = function(){
  cvs.onmousemove = null;
  x = 0;
  y = 0;
};

window.onblur = function(){
  ctx.closePath();
  ctx.beginPath();
  ctx.clearRect(0,0,1200,900)
};