让ie支持html5 placeholder

;$(function(){
var doc = document,
inputs = doc.getElementsByTagName('input'),
/*判断浏览器是否支持placeholder*/
supportPlaceholder = 'placeholder' in doc.createElement('input'),
placeholder = function(input) {
var text = input.getAttribute('placeholder'),
defaultValue = input.defaultValue;
if (defaultValue == '') {
input.value = text
}
input.onfocus = function() {
if (input.value === text) {
this.value = ''
}
};
input.onblur = function() {
if (input.value === '') {
this.value = text
}
}
};
if (!supportPlaceholder) {
for (var i = 0,
len = inputs.length; i < len; i++) {
var input = inputs[i],
text = input.getAttribute('placeholder');
if (input.type === 'text' && text) {
placeholder(input)
}
}
}

});