var arr = ["19998321312.jpg","19998321312.jpg","19998321312.jpg","19998321312.jpg","19998321312.jpg","19998321312.jpg","19998321312.jpg"]
function arrimg() {

var arr2 = [];
 arr.forEach(function(ele,index){  
    arr2.push(arr[index].split('.').join('_'+parseInt(index+1)+'.'));  
  });  
return arr2;

}

你這個(gè)要傳值之前是一個(gè)數(shù)組嗎?
function handleSort () {
let arr = [19998321312,19998321312,19998321312,19998321312,19998321312,19998321312,19998321312,19998321312,19998321312,19998321312]
let result = []
for (let i = 0; i < arr.length; i++) {

if (i <= 9) {
  arr[i] = arr[i] + '_0' + (i*1+1)
  result.push(arr[i])
} else {
  arr[i] = arr[i] + '_' + i
  result.push(arr[i])
}

}
return result
}
//handleSort()
是的話可以試試

  var arr = [{url: '19998321312.jpg'}];
  var result = arr.map((item,index) => {
      var strIndex = index + 1;
      if((strIndex).toString().length===1){
        strIndex = '0' + strIndex;
      }
      var urlIndex = item.url.lastIndexOf('.');
      var beforeUrl = item.url.slice(0, urlIndex);
      var afterUrl = item.url.slice(urlIndex)
      item.url = beforeUrl + '_' + strIndex + afterUrl;
      return item;
  });
  console.log(result); // [{url: "19998321312_01.jpg"}]
  // 多個(gè)改下數(shù)組arr即可。

'19998321312.jpg'.replace(/(\d+)(\.jpg)/g,'$1'+'_01'+'$2')
//"19998321312_01.jpg"