Untitled

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width">
  <title>JS Bin</title>
</head>
<body>

<script id="jsbin-javascript">
var res = [
  {
    photos:'1,2,3,4,5'
  },
  {
    photos:'5,6,7,8,9'
  }
]

res.forEach((item)=>{
  item.photos = item.photos.split(',')
//   item.photos.forEach((childItem)=>{
//     childItem = '1' + childItem;
//   })
  for(let i =0;i<item.photos.length;i++){
    item.photos[i] = '1'+item.photos[i]
  }
})
console.log(res);
</script>


<script id="jsbin-source-javascript" type="text/javascript">var res = [
  {
    photos:'1,2,3,4,5'
  },
  {
    photos:'5,6,7,8,9'
  }
]

res.forEach((item)=>{
  item.photos = item.photos.split(',')
//   item.photos.forEach((childItem)=>{
//     childItem = '1' + childItem;
//   })
  for(let i =0;i<item.photos.length;i++){
    item.photos[i] = '1'+item.photos[i]
  }
})
console.log(res);</script></body>
</html>