Get the highest and lowest values of a certain attribute in jQuery or Javascript

function minMaxId(selector) {
  var min=null, max=null;
  $(selector).each(function() {
    var id = parseInt(this.id, 10);
    if ((min===null) || (id < min)) { min = id; }
    if ((max===null) || (id > max)) { max = id; }
  });
  return {min:min, max:max};
}

minMaxId('div'); // => {min:1, max:5}

http://jsfiddle.net/qQvVQ/

To achieve this you can create an array containing all the id values, then use Math to get the highest/lowest:

var ids = $('.maindiv[id]').map((i, el) => parseInt(el.id, 10)).get();
var lowest = Math.min.apply(Math, ids); // = 1
var highest = Math.max.apply(Math, ids); // = 5

console.log(`${lowest} => ${highest}`);

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<div id="2" class="maindiv">test</div>
<div id="5" class="maindiv">test</div>
<div id="3" class="maindiv">test</div>
<div id="1" class="maindiv">test</div>
<div class="maindiv">test</div>
<div id="4" class="maindiv">test</div>

Note the [id] attribute selector is required, otherwise 0 is assumed for the missing value.

If you need IE support, you need to use an anonymous function instead of the arrow function:

var ids = $(".maindiv[id]").map(function() {
  return parseInt(this.id, 10);
}).get();