js实现占位符功能，类似python format函数

String.prototype.format= function() {
            if(arguments.length === 0) return this;
            var param = arguments[0], str= this;
            if(typeof(param) === 'object') {
                for(var key in param)
                    str = str.replace(new RegExp("\\{" + key + "\\}", "g"), param[key]);
                return str;
            } else {
                for(var i = 0; i < arguments.length; i++)
                    str = str.replace(new RegExp("\\{" + i + "\\}", "g"), arguments[i]);
                return str;
            }
        }

console.log("abc{0}，abc{1}".format("123","234"));