Edit in JSFiddle

//Searching for a substring

let str = "As sly as a fox, as strong as an ox";
let target = "as";

let pos = -1;
while ((pos = str.indexOf(target, pos + 1)) != -1) {

document.getElementById("result").innerHTML =  pos;

document.getElementById("result").innerHTML += '<br />';
}

<h1>String</h1>

<h2>Searching for a substring</h2>

<blockquote>
let str = "As sly as a fox, as strong as an ox";
let target = "as";<br />
result:<br />
<span id="result"></span>
</blockquote>