JS簡単なキャンバスの絵図の例を実現します.

 
  
var startX; 
  
var startY; 
  
var endX; 
  
var endY; 
  
var radius; 
  
var canvas = document.getElementById("myCanvas"); 
  
var context = canvas.getContext("2d"); 
  
var select = document.getElementsByTagName("select"); 
  
var startX; 
  
var startY; 
  
var endX; 
  
var endY; 
  
var radius; 
  
var canvas = document.getElementById("myCanvas"); 
  
var context = canvas.getContext("2d"); 
  
var select = document.getElementsByTagName("select"); 
  
 
 

 
  
window.onload=function() { 
  
    canvas.onmousedown = function(e) { 
  
    e = e || event; 
  
    startX = e.clientX; 
  
    startY = e.clientY; 
  
    if(select[0].value == "arc") { 
  
        canvas.onmousemove = moveShowArc; 
  
    } else { 
  
        canvas.onmousemove = moveShowRect; 
  
    } 
  
    } 
  
    canvas.onmouseup = function() { 
  
    canvas.onmousemove = ""; 
  
    } 
  
} 
  
function moveShowRect(e) { 
  
    context.clearRect(0, 0, 500, 300); 
  
    endX = e.clientX - startX; 
  
    endY = e.clientY - startY; 
  
    context.beginPath(); 
  
    context.rect(startX, startY, endX, endY); 
  
    context.fillStyle = "#8ED6FF"; 
  
    context.fill(); 
  
    context.lineWidth = 3; 
  
    context.strokeStyle = "black"; 
  
    context.stroke(); 
  
} 
  
function moveShowArc(e) { 
  
    context.clearRect(0, 0, 500, 300); 
  
    endX = e.clientX - startX; 
  
    endY = e.clientY - startY; 
  
    radius = Math.sqrt(Math.pow(endX,2)+Math.pow(endY,2)); 
  
    context.beginPath(); 
  
    context.arc(startX, startY,radius,0,2 * Math.PI,false); 
  
    context.fillStyle = "#8ED6FF"; 
  
    context.fill(); 
  
    context.lineWidth = 3; 
  
    context.strokeStyle = "black"; 
  
    context.stroke(); 
  
} 
  
window.onload=function() { 
  
    canvas.onmousedown = function(e) { 
  
 e = e || event; 
  
 startX = e.clientX; 
  
 startY = e.clientY; 
  
 if(select[0].value == "arc") { 
  
     canvas.onmousemove = moveShowArc; 
  
 } else { 
  
     canvas.onmousemove = moveShowRect; 
  
 } 
  
    } 
  
    canvas.onmouseup = function() { 
  
 canvas.onmousemove = ""; 
  
    } 
  
} 
  
function moveShowRect(e) { 
  
    context.clearRect(0, 0, 500, 300); 
  
    endX = e.clientX - startX; 
  
    endY = e.clientY - startY; 
  
    context.beginPath(); 
  
    context.rect(startX, startY, endX, endY); 
  
    context.fillStyle = "#8ED6FF"; 
  
    context.fill(); 
  
    context.lineWidth = 3; 
  
    context.strokeStyle = "black"; 
  
    context.stroke(); 
  
} 
  
function moveShowArc(e) { 
  
    context.clearRect(0, 0, 500, 300); 
  
    endX = e.clientX - startX; 
  
    endY = e.clientY - startY; 
  
    radius = Math.sqrt(Math.pow(endX,2)+Math.pow(endY,2)); 
  
    context.beginPath(); 
  
    context.arc(startX, startY,radius,0,2 * Math.PI,false); 
  
    context.fillStyle = "#8ED6FF"; 
  
    context.fill(); 
  
    context.lineWidth = 3; 
  
    context.strokeStyle = "black"; 
  
    context.stroke(); 
  
}