function getPath()
{
  //判断浏览器
  var Sys = {};
  var obj = document.getElementById("headImg");
  var viewer = document.getElementById("viewImg");
  var ua = navigator.userAgent.toLowerCase(); 
  var s; 
  (s = ua.match(/msie ([\d.]+)/)) ? Sys.ie = s[1] : 
  (s = ua.match(/firefox\/([\d.]+)/)) ? Sys.firefox = s[1] : 
  (s = ua.match(/chrome\/([\d.]+)/)) ? Sys.chrome = s[1] : 
  (s = ua.match(/opera.([\d.]+)/)) ? Sys.opera = s[1] : 
  (s = ua.match(/version\/([\d.]+).*safari/)) ? Sys.safari = s[1] : 0;
  var path="";
  if(Sys.ie<="6.0"){
    //ie5.5,ie6.0
    viewer.src = http://www.mamicode.com/obj.value;
  }else if(Sys.ie>="7.0"){
    //ie7,ie8
    obj.select();
    //加上这一句防止报告安全问题
    obj.blur();
    viewer.src = http://www.mamicode.com/document.selection.createRange().text;
  }else if(Sys.firefox){
    if (obj.files) {
        var accept = ["image/png","image/jpeg","image/jpg","image/gif"];
        if (accept.indexOf(obj.files[0].type) > -1) {
            var reader = new FileReader();
            reader.readAsDataURL(obj.files[0]);
            //延迟一会等待文件读取完毕
            var t = setTimeout(function(){
            viewer.src = http://www.mamicode.com/reader.result;
            clearTimeout(t);
            },100)
        }
    }
    else
    {
        viewer.src = http://www.mamicode.com/obj.value;
    }
  }else if(Sys.chrome){
    viewer.src = http://www.mamicode.com/obj.value;
  }
}

<form>
<input type="file" id="headImg" />
<input id="btnOk" type="button" value="http://www.mamicode.com/ok" onclick="getPath()"/>
<img src="http://blog.163.com/zhchf_52@yeah/blog/" alt="view" id="viewImg" />
</form>

另一种：

var obj = document.getElementById("headImg");
  var viewer = document.getElementById("viewImg");

js获取input file完整路径的方法