Since in javascript the + operator is used for both concatenation and addition, you need to put i+1 in a bracket, so that the first index gets increased and then you add it to p generate a unique id.

“p” + (i+1) will work

Replace:

sI[i].id = "p" + i + 1;

with:

sI[i].id = "p" + (i + 1);

If you want to do it more gracefully, you can use this

document.querySelectorAll("#article p").forEach(function(node,i){
    node.id = 'p'+(i+1)
})

Adding brackets to isolate your addition will add and not concatenate

like so "p" + (i + 1)

var i;
var sI = document.querySelectorAll("#article p");

for (i = 0; i < sI.length; i++) {

  sI[i].id = "p" + (i + 1);

}

<div id="article">
  <p>1</p>
  <p>2</p>
  <p>3</p>
  <p>4</p>
  <p>5</p>
  <div>

i have some <p> inside <div>, how to set ID for all of them using for loop?

HTML

<div id="article">
    <p>1</p>
    <p>2</p>
    <p>3</p>
    <p>4</p>
    <p>5</p>
<div>

the result will be like that:

<div id="article">
    <p id="p1">1</p>
    <p id="p2">1</p>
    <p id="p3">1</p>
    <p id="p4">1</p>
</div>

i made this code but not working

var i;
var sI = document.querySelectorAll("#article p");

for (i = 0; i < sI.length; i++){

    sI[i].id = "p" + i + 1;
     
}