怎样使一个INPUT框里的文字在框被点击后自动全选或清除？

$("#smsContent").focus(function(){
    this.select();
});

<input name="keywords" type="text" value="关键字" οnfοcus="if(this.value == this.defaultValue) this.value = ''" οnblur="if(this.value == '') this.value = this.defaultValue">