document.title = '方眼紙';

var style = document.createElement('style');
style.innerText = 'html,body{overflow:hidden;cursor:crosshair;}';
document.getElementsByTagName('head').item(0).appendChild(style);

var canvas = document.createElement('canvas');
var w = document.body.clientWidth;
var h = document.body.clientHeight;
var min = Math.min(w,h);
canvas.setAttribute('width',w);
canvas.setAttribute('height',h);
let demo = document.getElementById('demo');
demo.appendChild(canvas);

var ctx = canvas.getContext('2d');
var cx = w/2;
var cy = h/2;
var t = 0;
var max = 40;
var iw = min/max;
var ih = min/max;

function draw() {
  ctx.clearRect(0, 0, w, h);
  var ch = Math.floor(Math.random()*360);
  var color = 'hsl('+ch+',100%,50%)';
  ctx.fillStyle = color;
  ctx.setLineDash([2, 2]);
  for(i=0;i<w/iw;i++){
    if(i%2 === 0){
      ctx.strokeStyle = 'rgba(0,0,0,0.3)';
    } else {
      ctx.strokeStyle = 'rgba(0,0,0,0.1)';
    }
    ctx.beginPath();
    ctx.moveTo(i*iw, 0);
    ctx.lineTo(i*iw, h);
    ctx.stroke();
  }
  for(j=0;j<h/ih;j++){
    if(j%2 === 0){
      ctx.strokeStyle = 'rgba(0,0,0,0.3)';
    } else {
      ctx.strokeStyle = 'rgba(0,0,0,0.1)';
    }
    ctx.beginPath();
    ctx.moveTo(0, j*ih);
    ctx.lineTo(w, j*ih);
    ctx.stroke();
  }
}
draw();
/*
setInterval(function(){
  draw();
},9);
*/





document.oncontextmenu = function () {return false;}
function resize() {
  canvas.width = window.innerWidth;
  canvas.height = window.innerHeight;
  console.table({w:canvas.width, h:canvas.height});
  w = document.body.clientWidth;
  h = document.body.clientHeight;
  cx = w/2;
  cy = h/2;
}

window.onresize = resize;
window.addEventListener('click', function() {
  resize();
  document.body.requestFullscreen();
}, false );

document.addEventListener('fullscreenchange',function(e){
  resize();
});

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view-source:https://hi0a.com/demo/-js/js-canvas-grid-graph-paper/