How to skip a failed api lookup and pass null parameters instead?

Question

If the "Title" parameter isn't passed via the url, then my initial api call fails, but even failure, I need it continue to the my item save routine.

Unfortunately, an error is thrown on siteIdFromParameter = data.d.results[0].Id (because there isn't a parameter to execute the query against). In that situation, how would I just skip that "failure" and then pass null values for SiteUrl and SiteID?

function getUrlParam(parameter, defaultvalue){
    var urlparameter = defaultvalue;
    if(window.location.href.indexOf(parameter) > -1){
        urlparameter = getUrlVars()[parameter];
    }
    return urlparameter;
}

function addFeatureSuggestion() {

   var siteUrlFromParameter = getUrlParam('Title', _spPageContextInfo.webAbsoluteUrl);

    $.ajax({
     url: _spPageContextInfo.webAbsoluteUrl + "/_api/web/lists/getbytitle('" + listName2 + "')/items?$filter=SitePath eq '/sites/" + siteUrlFromParameter + "'",

        type: "GET",
        contentType: "application/json;odata=verbose",
        async: false,
        headers: {
           "Accept": "application/json;odata=verbose",
           "X-RequestDigest": $("#__REQUESTDIGEST").val()
        },
    }).then(function (data) {
        siteIdFromParameter = data.d.results[0].Id

        item = {
        "__metadata": { "type": "SP.Data.FeatureSuggestionsListItem" },
        "SiteURL": siteUrlFromParameter,
        "SiteId": siteIdFromParameter,
        "Title": $('#inputTitle').val(),
        "Suggestion": $('#inputSuggestion').val()
        };
    });
}

Answer 1

In the case the SiteUrl is nonexistent or invalid, your then promise resolution should still fire, but the data parameter passed to it will likely either contain an error object, or at least not contain the siteID your were trying to retrieve. Inside your then, create your item without the Site parameters, then inspect each sub-level of the returned data (for truth-iness) to determine whether you got good results, and only add in the Site values if you have a good result set:

function addFeatureSuggestion() {
    var siteUrlFromParameter = getUrlParam('Title', _spPageContextInfo.webAbsoluteUrl);

    $.ajax({
     url: _spPageContextInfo.webAbsoluteUrl + "/_api/web/lists/getbytitle('" + listName2 + "')/items?$filter=SitePath eq '/sites/" + siteUrlFromParameter + "'",


        type: "GET",
        contentType: "application/json;odata=verbose",
        async: false,
        headers: {
          "Accept": "application/json;odata=verbose",
         "X-RequestDigest": $("#__REQUESTDIGEST").val()
        },
    }).then(function (data) {
        item = {
            "__metadata": { "type": "SP.Data.FeatureSuggestionsListItem" },
            "Title": $('#inputTitle').val(),
            "Suggestion": $('#inputSuggestion').val()
        };

        if (data && data.d && data.d.results && data.d.results.length && data.d.results[0].Id) {
            siteIdFromParameter = data.d.results[0].Id

            item.SiteURL = siteUrlFromParameter;
            item.SiteId = siteIdFromParameter;
        }

        // do something with `item` here
    });
}