placeholder js简单实现

1  window.onload = function() {
2          var input = document.getElementById("input");
3           input.onblur = function() {
4                   this.value = (this.value == '' ? this.defaultValue : this.value);
5               };
6                 input.onfocus = function() {
7                 this.value = (this.value == this.defaultValue ? '' : this.value);
8             };
9  };