为什么这么些后面是个undefined

Question

alert('2010061800'.replace(/(\d{4})(\d{2})(\d{2})(\d{2})/,function($0,$1,$2,$3,$4){
return $1+'-'+$2+'-'+$3+'　'+({00:'上午','01':'下午'}[$4]);
}));

Answer 1

alert('2010061800'.replace(/(\d{4})(\d{2})(\d{2})(\d{2})/,function($0,$1,$2,$3,$4){
    return $1+'-'+$2+'-'+$3+'　'+({'00':'上午','01':'下午'}[$4]);
}));
00====>'00'
数字的属性别用数字  用字符串

Answer 2

  var  arr = "{ '00': '上午', '01': '下午' }";
        alert('2010061800'.replace(/(\d{4})(\d{2})(\d{2})(\d{2})/, function($0, $1, $2, $3, $4) {
            return $1 + '-' + $2 + '-' + $3 + '　' + eval("(" + arr+ ")")[$4];
        }));