When it comes to obtaining an array of classes, there isn't a direct method available, although the native DOM classList can be somewhat similar. However, relying solely on classList may not necessarily simplify things.
One approach could be:
var xyz = $(".datacheck .classic_div_data").attr("class").match(/\bcustomdataid_(\d+)\b);
xyz = xyz && xyz[1];
This regex efficiently captures the numeric part of the class without being overly sensitive (e.g., to the order of classes within the list).
For instance:
var xyz = $(".datacheck .classic_div_data").attr("class").match(/\bcustomdataid_(\d+)\b/);
xyz = xyz && xyz[1];
console.log("xyz = '" + xyz + "'");
<div class="datacheck">
<div class="classic_div_data customdataid_305">
some values appear here
</div>
<div class="optiondiv">
</div>
</div>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
If modifying the HTML is an option, using a data-* attribute instead of a class might be more suitable:
<div class="classic_div_data" data-custom-id="305">
Subsequently:
var xyz = $(".datacheck [data-custom-id]").attr("data-custom-id");
Example:
var xyz = $(".datacheck [data-custom-id]").attr("data-custom-id");
console.log("xyz = '" + xyz + "'");
<div class="datacheck">
<div class="classic_div_data" data-custom-id="305">
some values come here
</div>
<div class="optiondiv">
</div>
</div>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>