check whether the value is selected in the multi-select field or not using search. lookup.

The following code can be used to check whether the value is selected in the multi-select field or not using search. lookup.

var atReclookup = search.lookupFields({
    type: 'customrecord_ncfar_assettype',
    id: assetType,
    columns: ['custrecord_assettype_subsidiary', 'custrecord_assettypeassetacc']
});
var atSubsidary;
var atAssetAccount;
var assetSubValue = atReclookup.custrecord_assettype_subsidiary;
var typeArray = Array.isArray(assetSubValue);
if (atReclookup.custrecord_assettypeassetacc.length > 0) {
    atAssetAccount = atReclookup.custrecord_assettypeassetacc[0].value;
}
if ((checkForParameter(account) == true) && (checkForParameter(atAssetAccount) == true)) {
    if (account != atAssetAccount) {
        alert("Selected Asset type's Asset Account is different from Bill/journal's Account")
        return false;
    }
}

if (typeArray == true) {
    if (atReclookup.custrecord_assettype_subsidiary.length > 0) {
        atSubsidary = atReclookup.custrecord_assettype_subsidiary[0].value;
    }
    if (checkForParameter(atSubsidary) == true) {
        if (atSubsidary.length <= 0) {
            alert("Asset Type has no Subsidiary value. Asset type should have a subsidiary value and it should be the same as Bill/journal ")
            return false
        }
        if (atSubsidary.length > 0) {
            if (atSubsidary.includes(recSubsidary) == false) {
                alert("Subsidiary of Asset type and Bill/Journal should be the same")
                return false;
            }
        }
    }
    if (checkForParameter(atSubsidary) == false) {
        alert("Selected Asset Type hasn't any Subsidiary")
        return false
    }
} else {
    //subsidiary is multiselect subsidiary
    var assetsubsidiary = assetSubValue.value;
    var atsubArray = assetsubsidiary.split(',');
    if (atsubArray.length > 0) {
        var subexist = atsubArray.indexOf(recSubsidary);
        if (subexist == -1) {
            alert("Subsidiary of Asset type and Bill/Journal should be the same")
            return false
        }
    }
}