html怎么显示直线,html怎么用鼠标画出一条直线，鼠标移动时候要能看到线条

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window.onload = function(){

var oC = document.getElementById('c1');

var oGC = oC.getContext('2d');

oC.onmousedown = function(ev){

var ev = ev || window.event;

oGC.beginPath();

oGC.moveTo(ev.clientX-oC.offsetLeft,ev.clientY-oC.offsetTop);

document.onmousemove = function(ev){

var ev = ev || window.event;

// oGC.lineTo(ev.clientX-oC.offsetLeft,ev.clientY-oC.offsetTop);

// oGC.closePath();

// oGC.stroke();

};

document.onmouseup = function(){

var ev = ev || window.event;

oGC.lineTo(ev.clientX-oC.offsetLeft,ev.clientY-oC.offsetTop);

oGC.stroke();

document.onmousemove = null;

document.onmouseup = null;

};

};

};