placeholder属性是HTML5新添加的属性，当input或者textarea设置了该属性后，该值的内容将作为灰色提示显示在文本框中，当文本框获得焦点时，提示文字消失，placeholder可作为输入框的提示文案
placeholder是常用的属性，它使得input框内有很友好的提示效果。高版本浏览器都支持placeholder属性，但IE9以下版本的浏览器并不支持这一属性。这里用JavaScript实现添加对浏览器的兼容处理。
方案一：jquery实现
（当浏览器不支持placeholder属性时，克隆一个和界面相同的input框，将placeholder的值设置为其value值，覆盖在界面input框所在位置，并将界面上的input隐藏掉）
调用方法： $(#selector).placeholder();(selector泛指css 的 id选择器)
（这里有一个问题，当文本框type=password时，引用此placeholder方案会使暗文密码变成明文密码）
;(function(window, document, $) {
// Opera Mini v7 doesn’t support placeholder although its DOM seems to indicate so
var isOperaMini = Object.prototype.toString.call(window.operamini) == '[object OperaMini]';
var isInputSupported = 'placeholder' in document.createElement('input') && !isOperaMini;
var isTextareaSupported = 'placeholder' in document.createElement('textarea') && !isOperaMini;
var prototype = $.fn;
var valHooks = $.valHooks;
var propHooks = $.propHooks;
var hooks;
var placeholder;
if (isInputSupported && isTextareaSupported) {
placeholder = prototype.placeholder = function() {
return this;
};
placeholder.input = placeholder.textarea = true;
} else {
placeholder = prototype.placeholder = function() {
var $this = this;
$this
.filter((isInputSupported ? 'textarea' : ':input') + '[placeholder]')
.not('.placeholder')
.bind({
'focus.placeholder': clearPlaceholder,
'blur.placeholder': setPlaceholder
})
.data('placeholder-enabled', true)
.trigger('blur.placeholder');
return $this;
};
placeholder.input = isInputSupported;
placeholder.textarea = isTextareaSupported;
hooks = {
'get': function(element) {
var $element = $(element);
var $passwordInput = $element.data('placeholder-password');
if ($passwordInput) {
return $passwordInput[0].value;
}
return $element.data('placeholder-enabled') && $element.hasClass('placeholder') ? '' : element.value;
},
'set': function(element, value) {
var $element = $(element);
var $passwordInput = $element.data('placeholder-password');
if ($passwordInput) {
return $passwordInput[0].value = value;
}
if (!$element.data('placeholder-enabled')) {
return element.value = value;
}
if (value == '') {
element.value = value;
// Issue #56: Setting the placeholder causes problems if the element continues to have focus.
if (element != safeActiveElement()) {
// We can't use `triggerHandler` here because of dummy text/password inputs :(
setPlaceholder.call(element);
}
} else if ($element.hasClass('placeholder')) {
clearPlaceholder.call(element, true, value) || (element.value = value);
} else {
element.value = value;
}
// `set` can not return `undefined`; see http://jsapi.info/jquery/1.7.1/val#L2363
return $element;
}
};
if (!isInputSupported) {
valHooks.input = hooks;
propHooks.value = hooks;
}
if (!isTextareaSupported) {
valHooks.textarea = hooks;
propHooks.value = hooks;
}
$(function() {
// Look for forms
$(document).delegate('form', 'submit.placeholder', function() {
// Clear the placeholder values so they don't get submitted
var $inputs = $('.placeholder', this).each(clearPlaceholder);
setTimeout(function() {
$inputs.each(setPlaceholder);
}, 10);
});
});
// Clear placeholder values upon page reload
$(window).bind('beforeunload.placeholder', function() {
$('.placeholder').each(function() {
this.value = '';
});
});
}
function args(elem) {
// Return an object of element attributes
var newAttrs = {};
var rinlinejQuery = /^jQuery\d+$/;
$.each(elem.attributes, function(i, attr) {
if (attr.specified && !rinlinejQuery.test(attr.name)) {
newAttrs[attr.name] = attr.value;
}
});
return newAttrs;
}
function clearPlaceholder(event, value) {
var input = this;
var $input = $(input);
if (input.value == $input.attr('placeholder') && $input.hasClass('placeholder')) {
if ($input.data('placeholder-password')) {
$input = $input.hide().next().show().attr('id', $input.removeAttr('id').data('placeholder-id'));
// If `clearPlaceholder` was called from `$.valHooks.input.set`
if (event === true) {
return $input[0].value = value;
}
$input.focus();
} else {
input.value = '';
$input.removeClass('placeholder');
input == safeActiveElement() && input.select();
}
}
}
function setPlaceholder() {
var $replacement;
var input = this;
var $input = $(input);
var id = this.id;
if (input.value == '') {
if (input.type == 'password') {
if (!$input.data('placeholder-textinput')) {
try {
$replacement = $input.clone().attr({ 'type': 'text' });
} catch(e) {
$replacement = $('<input>').attr($.extend(args(this), { 'type': 'text' }));
}
$replacement
.removeAttr('name')
.data({
'placeholder-password': $input,
'placeholder-id': id
})
.bind('focus.placeholder', clearPlaceholder);
$input
.data({
'placeholder-textinput': $replacement,
'placeholder-id': id
})
.before($replacement);
}
$input = $input.removeAttr('id').hide().prev().attr('id', id).show();
// Note: `$input[0] != input` now!
}
$input.addClass('placeholder');
$input[0].value = $input.attr('placeholder');
} else {
$input.removeClass('placeholder');
}
}
function safeActiveElement() {
// Avoid IE9 `document.activeElement` of death
// https://github.com/mathiasbynens/jquery-placeholder/pull/99
try {
return document.activeElement;
} catch (exception) {}
}
}(this, document, jQuery));
方案二： js/jQuery实现
实现思路：
1、首先判断浏览器是否支持placeholder属性，如果不支持则使用模拟placeholder
2、创建一个label标签：<label>密码</label> 标签里面的内容为所要添加的提示文字，该文字应该从对应的input|textarea标签取得其placeholder属性值，再将label标签遮盖 到所对应的input|textarea上
3、代码实现如下：
; (function (win) {
win.isPlaceholer = function () {
var input = document.createElement("input");
return "placeholder" in input;
};
win.placeholder = function () {
if (!isPlaceholer()) {
var Placeholder =function (obj) {
this.input = obj;
var te = obj.getAttribute('placeholder');
this.label = document.createElement('label');
this.label.innerHTML = te;
this.label.id = obj.id + 'Label';
this.label.style.cssText = 'position:absolute; text-indent:4px;color:#999999; font-size:14px;';
if (obj.value !== '') {
this.label.style.display = 'none';
}
this.init();
};
Placeholder.prototype = {
getxy: function (obj) {
var left, top;
if (document.documentElement.getBoundingClientRect) {
var html = document.documentElement,
body = document.body,
pos = obj.getBoundingClientRect(),
st = html.scrollTop || body.scrollTop,
sl = html.scrollLeft || body.scrollLeft,
ct = html.clientTop || body.clientTop,
cl = html.clientLeft || body.clientLeft;
left = pos.left + sl - cl;
top = pos.top + st - ct;
} else {
while (obj) {
left += obj.offsetLeft;
top += obj.offsetTop;
obj = obj.offsetParent;
}
}
return {
left: left,
top: top
};
},
getwh: function (obj) {
return {
w: obj.offsetWidth,
h: obj.offsetHeight
};
},
setStyles: function (obj, styles) {
for (var p in styles) {
obj.style[p] = styles[p] + 'px';
}
},
init: function () {
var label = this.label,
input = this.input,
getXY = this.getxy,
xy = this.getxy(input),
wh = this.getwh(input);
this.setStyles(label, { 'width': wh.w, 'height': wh.h, 'lineHeight': 40, 'left': xy.left + 8, 'top': xy.top });
document.body.appendChild(label);
label.onclick = function () {
this.style.display = "none";
input.focus();
};
input.onfocus = function () {
label.style.display = "none";
};
input.onblur = function () {
if (this.value === "") {
label.style.display = "block";
}
};
if (window.attachEvent) {
window.attachEvent("onresize", function () {
var xy = getXY(input);
Placeholder.prototype.setStyles(label, { 'left': xy.left + 8, 'top': xy.top });
});
} else {
window.addEventListener("resize", function () {
var xy = getXY(input);
Placeholder.prototype.setStyles(label, { 'left': xy.left + 8, 'top': xy.top });
}, false);
}
}
};
var inpColl = $("#Box input:visible");//这里是页面上要添加placeholder支持的input
//var inpColl = document.getElementsByTagName('input'),
var textColl = document.getElementsByTagName('textarea');//这里是页面上要添加placeholder支持的textarea
//var lableArr = $("#Box lable");
var toArray = function (coll) {
for (var i = 0, a = [], len = coll.length; i < len; i++) {
a[i] = coll[i];
}
return a;
};
var inpArr = toArray(inpColl),
textArr = toArray(textColl),
placeholderArr = inpArr.concat(textArr);
for (var i = 0; i < placeholderArr.length; i++) {
if (placeholderArr[i].getAttribute('placeholder') !== null) {
new Placeholder(placeholderArr[i]);
}
}
}
};
}(window));
方法执行后界面代码：（lable在页面上）
IE8效果图如下：
原文：http://www.cnblogs.com/meggie523/p/5067774.html
除了上面两种方式，还发现下面这段代码也是可以的，我在公司项目中用的就是下面这段，测试可用
// 兼容IE8,IE9下placeholder属性
if (!('placeholder' in document.createElement('input'))) {
$('input[placeholder],textarea[placeholder]').each(function() {
var that = $(this),
text = that.attr('placeholder');
if (that.val() === "") {
that.val(text).addClass('placeholder');
}
that.focus(function() {
if (that.val() === text) {
that.val("").removeClass('placeholder');
}
}).blur(function() {
if (that.val() === "") {
that.val(text).addClass('placeholder');
}
}).closest('form').submit(function() {
if (that.val() === text) {
that.val('');
}
});
});
}
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