javascript 文本框水印/占位符(watermark/placeholder)实现方法

<input type="text" id="text1" />
<script>
var el = document.getElementById("text1");
if (el.value == "")
el.value = "提示信息";

el.onfocus = function() {
if (this.value == "提示信息")
this.value = "";
};
el.onblur = function() {
if (this.value == "")
this.value = "提示信息";
}
</script>

<input type="text" />

<span style="position:relative;">
<span style="position:absolute;">提示信息</span>
<input type="text" />
</span>

<span style="position:absolute;">提示信息</span>
<input type="text" />

/* 标记的主要样式 style */
.w-label {
position: absolute;
padding: 0 0 0 6px;
margin: 0;
font-size: .8em;
color: #999;
opacity: 1;
}
/* 隐藏标记 */
.w-hide {
visibility: hidden;
opacity: 0;
}
/* 表单元素获得焦点时，标记的颜色 */
.w-active {
color: #ddd;
}

<span class="w-label">提示信息</span>
<input type="text" />

/* 事件绑定 */
var addEvent = document.addEventListener ?
function(element, type, fn) {
element.addEventListener(type, fn, false);
} :
function(element, type, fn) {
element.attachEvent("on" + type, fn);
},
/* 事件解除绑定 */
removeEvent = document.removeEventListener ?
function(element, type, fn) {
element.removeEventListener(type, fn, false);
} :
function(element, type, fn) {
element.detachEvent("on" + type, fn);
},
/* 文本框水印/占位符 */
watermark = function(element, text) {
if (!(this instanceof watermark))
return new watermark(element, text);
var place = document.createElement("span");//提示信息标记
element.parentNode.insertBefore(place, element);//插入到表单元素之前的位置
place.className = "w-label";
place.innerHTML = text;
place.style.height = place.style.lineHeight = element.offsetHeight + "px";//设置高度、行高以居中
element.place = this;
function hideIfHasValue() {
if (element.value && place.className.indexOf("w-hide") == -1)
place.className += " w-hide";
}
function onFocus() {
hideIfHasValue()
if (!element.value && place.className.indexOf("w-active") == -1)
place.className += " w-active";
}
function onBlur() {
if (!element.value) {
place.className = place.className.replace(" w-active", "");
place.className = place.className.replace(" w-hide", "");
}
}
function onClick() {
hideIfHasValue();
try {
element.focus && element.focus();
} catch (ex) {}
}
// 注册各个事件
hideIfHasValue();
addEvent(element, "focus", onFocus);
addEvent(element, "blur", onBlur);
addEvent(element, "keyup", hideIfHasValue);
addEvent(place, "click", onClick);
// 取消watermark
this.unload = function() {
removeEvent(element, "focus", onFocus);
removeEvent(element, "blur", onBlur);
removeEvent(element, "keyup", hideIfHasValue);
removeEvent(place, "click", onClick);
element.parentNode.removeChild(place);
element.place = null;
};
};

<input id="text1" type="text" />
<input type="button" id="button1" value="取消watermark" />
<script>
var m1 = watermark(document.getElementById("text1"), "提示信息");
addEvent(document.getElementById("button1"), "click", function() {
m1.unload();
});
</script>

var html5support = "placeholder" in document.createElement("input");

placeHolderForm = function(form) {
var ph, elems = form.elements,
html5support = "placeholder" in document.createElement("input");
if (!html5support) {
for (var i = 0, l = elems.length; i < l; i++) {
ph = elems[i].getAttribute("placeholder");
if (ph) elems[i].ph = watermark(elems[i], ph);
}
}
}

<form id="form2">
<fieldset>
<legend><strong>对不支持html5 placeholder的表单元素应用watermark</strong></legend>
<ul>
<li>
文本框：
<input type="text" placeholder="文本框文本框" />
</li>
<li>
密码框：
<input type="password" placeholder="密码框密码框" />
</li>
<li>
多行文本：
<textarea placeholder="多行文本多行文本"></textarea>
</li>
</ul>
</fieldset>
</form>
<script>
placeHolderForm(document.getElementById("form2"));
</script>