// Custom Validate script for text field
(function () {

    // Create an object to associate item number with item description
    var oItems = {};

    oItems["123"] = "file";
    oItems["124"] = "saw";

    // Get a reference to the txtItem field
    var f = getField("txtItem");

    // Set its value based on the entered value;
    if (event.value) {
        f.value = oItems[event.value] || "Unknown item";
    } else {
        f.value = "";
    }

})();