HTML 文本框获取焦点提示输入文字消失，失去焦点显示

function TextInfo(element, info) { 
    this._textBox = element;
    this.text = info;
    this._textBox.value = info;
    this._textBox.onblur = new createDelegate(this, this._onblur);
    this._textBox.onfocus = new createDelegate(this, this._onfocus); 
}
TextInfo.prototype = {
    _onblur: function () {
        if (this._textBox.value.length == 0) {
            this._textBox.value = this.text;
        }
    },
    _onfocus: function () { 
        if (this._textBox.value.length != 0 && this._textBox.value == this.text) {
            this._textBox.value = "";
        }
    }
} 
//方法委托，确保this关键字作用域正确。
createDelegate = function(instance, method) { 
    return function () {
　　　　　//指定方法调用者　
        return method.apply(instance, arguments);
    }
}

调用：new TextInfo(document.getElementById("textMarks"), "请输入用户名");