JS - Splitting a string and looping through results

As an alternative:

for(element of feedUpdateResponse.split("/")){
    do_your_thing();
}

Using for..in will end up giving you the indices on the array (keys), while for..on will give you the elements of the array (values).

You can also do:

for ([index, element] of Object.entries(feedUpdateResponse.split("/"))) {
    do_your_thing();
}

In the event that you need the index.

The disadvantage is it not being compatible with IE, but for personal projects or a quick automation script it usually does me plenty fine.

Get your array via string.split("/"). Iterate your array using your method of choice. I prefer Array.forEach():

feedUpdateResponse.split("/").forEach(function (item) {
    document.getElementById(item).style.visibility = "visible";
});

See the compatibility notes for using .forEach() in older browsers.

You can use .split() to split a string on a specified character with the results returned as an array. So then it's just a matter of looping through the array:

// given your existing variable
// feedUpdateResponse = "div1/div2/div3/div4" as set in the
// code in the question, add this:

var a = feedUpdateResponse.split("/"),
    i;

for (i = 0; i < a.length; i++) {
    document.getElementById("updateAvailable_" + a[i]).style.visibility
                                                                 = "visible";
}