JavaScript实现日期自动输入控件的完整代码与原理

<html>
<head>
<style>
<!-- 
.wr{font-size: 12pt; line-height: 22px}
.wr1 { FONT-SIZE: 12px; LINE-HEIGHT: 200%}
.wr2 { FONT-SIZE: 14px; LINE-HEIGHT: 200%}
.wr3 { FONT-SIZE: 12px}
.wr4 { FONT-SIZE: 12px; LINE-HEIGHT: 150%}
// --> 
</style>
<title>日期自动输入控件</title>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
</head>
<style type="text/css">
.date-picker-wp {
display: none;
position: absolute;
background: #f1f1f1;
left: 40px;
top: 40px;
border-top: 4px solid #3879d9;
} 
.date-picker-wp table {
border: 1px solid #ddd;
} 
.date-picker-wp td {
background: #fafafa;
width: 22px;
height: 18px;
border: 1px solid #ccc;
font-size: 12px;
text-align: center;
} 
.date-picker-wp td.noborder {
border: none;
background: none;
} 
.date-picker-wp td a {
color: #1c93c4;
text-decoration: none;
} 
.strong {font-weight: bold}
.hand {cursor: pointer; color: #3879d9}
</style>
<script type="text/javascript">
var DatePicker = function () {
var $ = function (i)
{ 
return document.getElementById(i)
}, 
addEvent = function (o, e, f) {o.addEventListener ? o.addEventListener(e, f, false) : o.attachEvent('on'+e, function(){f.call(o)})},
getPos = function (el) {
for (var pos = {x:0, y:0}; el; el = el.offsetParent) {
pos.x += el.offsetLeft;
pos.y += el.offsetTop;
} 
return pos;
}; 
var init = function (n, config) {
window[n] = this;
Date.prototype._fd = function () {var d = new Date(this); d.setDate(1); return d.getDay()};
Date.prototype._fc = function () {var d1 = new Date(this), d2 = new Date(this); d1.setDate(1); d2.setDate(1); d2.setMonth(d2.getMonth()+1); return (d2-d1)/86400000;};
this.n = n;
this.config = config;
this.D = new Date;
this.el = $(config.inputId);
this.el.title = this.n+'DatePicker';
this.update();
this.bind();
}; 
init.prototype = {
update : function (y, m) {
var con = [], week = ['Su','Mo','Tu','We','Th','Fr','Sa'], D = this.D, _this = this;
fn = function (a, b) {return '<td title="'+_this.n+'DatePicker" class="noborder hand" onclick="'+_this.n+'.update('+a+')">'+b+'</td>'},
_html = '<table cellpadding=0 cellspacing=2>';
y && D.setYear(D.getFullYear() + y);
m && D.setMonth(D.getMonth() + m);
var year = D.getFullYear(), month = D.getMonth() + 1, date = D.getDate();
for (var i=0; i<week.length; i++) con.push('<td title="'+this.n+'DatePicker" class="noborder">'+week[i]+'</td>');
for (var i=0; i<D._fd(); i++ ) con.push('<td title="'+this.n+'DatePicker" class="noborder"> </td>');
for (var i=0; i<D._fc(); i++ ) con.push('<td class="hand" onclick="'+this.n+'.fillInput('+year+', '+month+', '+(i+1)+')">'+(i+1)+'</td>');
var toend = con.length%7;
if (toend != 0) for (var i=0; i<7-toend; i++) con.push('<td class="noborder"> </td>');
_html += '<tr>'+fn("-1, null", "<<")+fn("null, -1", "<")+'<td title="'+this.n+'DatePicker" colspan=3 class="strong">'+year+'/'+month+'/'+date+'</td>'+fn("null, 1", ">")+fn("1, null", ">>")+'</tr>';
for (var i=0; i<con.length; i++) _html += (i==0 ? '<tr>' : i%7==0 ? '</tr><tr>' : '') + con[i] + (i == con.length-1 ? '</tr>' : '');
!!this.box ? this.box.innerHTML = _html : this.createBox(_html);
}, 
fillInput : function (y, m, d) {
var s = this.config.seprator || '/';
this.el.value = y + s + m + s + d;
this.box.style.display = 'none';
}, 
show : function () {
var s = this.box.style, is = this.mask.style;
s['left'] = is['left'] = getPos(this.el).x + 'px';
s['top'] = is['top'] = getPos(this.el).y + this.el.offsetHeight + 'px';
s['display'] = is['display'] = 'block';
is['width'] = this.box.offsetWidth - 2 + 'px';
is['height'] = this.box.offsetHeight - 2 + 'px';
}, 
hide : function () {
this.box.style.display = 'none';
this.mask.style.display = 'none';
}, 
bind : function () {
var _this = this;
addEvent(document, 'click', function (e) {
e = e || window.event;
var t = e.target || e.srcElement;
if (t.title != _this.n+'DatePicker') {_this.hide()} else {_this.show()}
}); 
}, 
createBox : function (html) {
var box = this.box = document.createElement('div'), mask = this.mask = document.createElement('iframe');
box.className = this.config.className || 'datepicker';
mask.src = 'javascript:false';
mask.frameBorder = 0;
box.style.cssText = 'position:absolute;display:none;z-index:9999';
mask.style.cssText = 'position:absolute;display:none;z-index:9998';
box.title = this.n+'DatePicker';
box.innerHTML = html;
document.body.appendChild(box);
document.body.appendChild(mask);
return box;
} 
}; 
return init;
}(); 
onload = function () {
new DatePicker('_DatePicker_demo', {
inputId: 'date-input',
className: 'date-picker-wp',
seprator: '-'
}); 
} 
</script>
<body bgcolor="#FFFFDB" text="#000000" leftmargin="0" topmargin="0" marginwidth="0" marginheight="0"> 
<form > 
 <table border="0" width="60%" align="center"> 
 <tr> 
 <td width="45%" class="wr4" align="right"> 生日：</td> 
 <td width="55%" class="wr4"> 
 <input type="text" name="mtime" id="date-input"> <font color="RED">*</font> 
 </td> 
 </tr> 
 <tr> 
 <td width="45%" align="right"><input type = "submit" value = "确定"/></td> 
 <td width="55%"><input type = "reset" value = "重置"/></td> 
 </tr> 
</table> 
</form> 
</body> 
</html>

页面加载与脚本执行顺序分析

HTML页面中JavaScript的执行顺序如下： 1）位于head标签内的脚本最先执行 2）位于body标签内的脚本随后执行 3）如果在body标签中注册了onload事件，则对应的JavaScript代码最后执行，即页面完全加载完毕后执行

注意：如果在body标签中注册了onload事件，而在head标签内引用了外部JavaScript文件，则该外部JS可能无法正常工作。建议将外部JS引用移至body内部或</html>标签之上，以确保正常引用。