js画图实例：anvas画图

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();
}