关于微信小程序过滤器filter的正确使用

var replaceStar = function (index) {
    var a
    switch (index) {
        case '1':
            a = '出生时'
            break;
        case '2':
            a = '1月龄'
            break;
        case '3':
            a = '2月龄'
            break;
        case '4':
            a = '3月龄'
            break;
        case '5':
            a = '4月龄'
            break;
        case '6':
            a = '5月龄'
            break;
        case '7':
            a = '6月龄'
            break;
        case '8':
            a = '8月龄'
            break;
        case '9':
            a = '9月龄'
            break;
        case '10':
            a = '1岁6月龄'
            break;
        case '11':
            a = '2岁'
           break;
        case '12':
            a = '3岁'
            break;
         case '13':
             a = '4岁'
            break;
        case '14':
            a = '6岁'
           break;
    }
    return a;
}
module.exports = {
    replaceStar: replaceStar
}

<wxs src="../../utils/filter.wxs" module="filter" />
<view class="time">{{filter.replaceStar(index)}}</view>