JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" maxlength="20" size="25" value="" name="zipc" id="zipc">
<input type="submit" value="Search">

JavaScript

jQuery('#zipc').focusout(function () {
    var _val = jQuery(this).val();
    var _array = ["BF1 3AA", "Harry"];
    for (var i = 0; i < _array.length; i++) {
        if (_val.indexOf(_array[i]) != -1) {
            alert('OH, snap! That\'s a BFPO postcode... We don\'t send stuff there. Bummer.');
            jQuery('#zipc').val("");
        }
        else {
        alert('Sorry');
            jQuery('#zipc').val("");
        }
    }
});