javascript占位符

传统javascript：

The text is inserted in the value attribute. On focus, it checks if the value is "search" and returns empty to clear the field. If the value is empty, it returns "search." As you can see, this way is not an efficient way because each field has to be checked.

<input type="text" value="Search" onfocus="if (this.value == 'Search') {this.value = '';}" onblur="if (this.value == '') {this.value = 'Search';}">

Now with HTML5 placeholder, it is more semantic to use placeholder than value attribute. However, placeholder text is not supported by all browsers. To make it cross-browser, Modernizr and jQuery come in handy here.

Modernizr is used here to check if placeholder is supported. If placeholder is not supported, the jQuery code will run. It finds all elements with placeholder attribute and stores in a variable. It then compares the input value with the placeholder attribute. If the input value is empty, it will display the placeholder text and add a "placeholder" class to the input field. View Demo.

To use this on your site, download a copy of Modernizr and jQuery and paste the following code any where in your html page (be sure the jquery.js and modernizr.js file is in correct path).

<script src="jquery.js"></script>
<script src="modernizr.js"></script>

<script>
$(document).ready(function(){

if(!Modernizr.input.placeholder){

    $('[placeholder]').focus(function() {
      var input = $(this);
      if (input.val() == input.attr('placeholder')) {
        input.val('');
        input.removeClass('placeholder');
      }
    }).blur(function() {
      var input = $(this);
      if (input.val() == '' || input.val() == input.attr('placeholder')) {
        input.addClass('placeholder');
        input.val(input.attr('placeholder'));
      }
    }).blur();
    $('[placeholder]').parents('form').submit(function() {
      $(this).find('[placeholder]').each(function() {
        var input = $(this);
        if (input.val() == input.attr('placeholder')) {
          input.val('');
        }
      })
    });

}

</script>

via:http://webdesignerwall.com/tutorials/cross-browser-html5-placeholder-text

另外一个原生的javascript实现；

function add_placeholder (id, placeholder)
{
    var el = document.getElementById(id);
    el.placeholder = placeholder;
 
    el.onfocus = function ()
    {
        if(this.value == this.placeholder)
        {
            this.value = '';
            el.style.cssText  = '';
        }
    };
 
    el.onblur = function ()
    {
        if(this.value.length == 0)
        {
            this.value = this.placeholder;
            el.style.cssText = 'color:#A9A9A9;';
        }
    };
 
    el.onblur();
}
 
// Add right before </body> or inside a DOMReady wrapper
add_placeholder('myInputField', 'IE Placeholder Text');

<inputtype="text"name="myInputField"value=""placeholder="HTML5 Placeholder Text"id="myInputField">

参考：https://gist.github.com/jaywilliams/1105055#file-example-html