jQuery plugin that clears and restores a field's default value

jquery.has_default_value.js

// clears and restores a field's default value
// example usage (js): $('input.has_default').hasDefaultValue();
// example usage (html): <input class="has_default" default="This is displayed by default" type="text"/>

jQuery.fn.hasDefaultValue = function() {
  function supports_input_placeholder() {
    var i = document.createElement('input');
    return 'placeholder' in i;
  }
  if(!supports_input_placeholder()){
    this.each(function(){
      if(this.value === ""){
        this.value = $(this).attr("placeholder")
      }
    });
    this.focus(function(event){
      if(this.value === $(this).attr("placeholder")){
        this.value = ""
      }
    })
    this.blur(function(event){
      if($(this).attr("placeholder") && this.value === ""){
        this.value = $(this).attr("placeholder");
      } 
    })
  }
};