QueryselectorAll

document.querySelectorAll(".select").forEach((item, index) => { // here
      item.addEventListener('click', arrow => {
      document.body.style.backgroundImage = `url("${background_list[index]}")`
})})

atau juga bisa

document.forms['fmtarget'].querySelectorAll('input[name^="a"]').forEach(function(el){el.value=el.name$(el).addClass("intro")})

atau juga bisa

var ct=0
document.forms[1].querySelectorAll('input').forEach(function(el){ct++el.type='text' el.value="[a|"+el.name+"]"+ct})

or

document.selectorAll('div').forEach(div => {
    div.addEventListener('click', () => {
        alert(div.innerHTML)
    })
})

or fill from json object

$.each(response, function( key, value ) {
			try {
				fmtarget["a["+key+"]"].value=value
			}
			catch(err) {console.log("a["+key+"]"+err.message)}
		})

document.querySelectorAll(".btnsave").forEach(el => el.addEventListener('click', event => {
	var elrow=el.parentNode.parentNode.parentNode.parentNode
	$.post('?act=save',$(elrow.querySelectorAll('input')).serialize()).done(function(response){
		console.log(response)
	})
}))

pilihan lain

<form method='post'>
		Kategori Tagihan
		<? mkslcrs('kodecategory','',"select kode,nama from chp_bill_category where owner='$user->owner' order by kode ASC"," onchange='gantilabel(this)'",'Kategori Tagihan')?>
		<script>
		function gantilabel(o){
			$("#txtlabel").val(o.options[o.selectedIndex].text + ' Bln <? echo date("m-Y")?>' )
		}
		</script>
		Target Kelas
		<? mkslcrs('idkelas','',"select idx,kelas from chp_kelas where owner='$user->owner' and ta='$user->tahun' order by nmr ASC",'','Target Kelas')?>
		Jenis Kelamin
		<? mkslc('kelamin','',array('L'=>'Laki-laki','P'=>'Perempuan'))?>
		Label Tagihan
		<input type="text"  class="form-control" id='txtlabel' name='a[label]' />
		Nominal Tagihan<? mktxt('angka')?>
		<div class="btn-action" style='margin-top:10px'><button type='submit' class='btn btn-primary' onclick="process(this.form) return false">Buat Tagihan Masal</button></div>
	  </form>
	<script>
  		  function process(o){
			  //var elements = o.querySelectorAll("input[type=text][value='']")
			var err=0
			o.querySelectorAll(".form-control").forEach(function(el) {
				el.style.backgroundColor = ""
				if(el.value==''){
					el.style.backgroundColor = "yellow"
					err='1'
				}
			})
			if(err>0){alert('semua kotak harus terisi')return}
			$.post('?act=bill-process',$(o).serialize()).done(function(response){
				$("#viewplace").html("<div class='card' style='padding:10px'>"+response+"</div>")
				var objDiv = document.getElementById("viewplace")
				objDiv.scrollTop = objDiv.scrollHeight
			}) 
			return false
		  }
		  function posting(){
			  $.post('?act=bill-post').done(function(response){
				  console.log(response)
				  alert("Tagihan telah ditambahkan")
				  location.reload()
			  })
		  }
			</script>