Get list item data from different site collection using REST api in Javascript

Question

I am trying to retrieve list item data from different site collection in SP2013.

I am storing all site URL in an array and I am iterating this using for loop. while in for loop I have called another method which has REST code and I am passing site URL one by one to REST.

However this is not working as for loop keeps incrementing and REST is executing for only 1 site URL and giving me data from first site only. I think this is happening because of Async call.

SiteURL[] has 3 url and I want to get data from this 3 sites.

Code sample:

function getSubmissionNo(siteURL)
{        
   var currentNewUrl;
   for(a=0;a<=siteURL.length;a++)
   {            
      currentNewUrl=siteURL[a];  
      getDataUsingRest(currentNewUrl).done(function(result){})
   }    
}
function getDataUsingRest(requiredUrl)
{
   var dfd=new $.Deferred;
   $.ajax({
        url: requiredUrl+"/_api/web/lists/getbytitle('E-files')/items?$select=ImgURL,FileRef&$expand=ContentType&$filter=ContentType eq 'Submission Folder'",
        type: "GET",  
        asyn: false,            
        headers: {
            "Accept": "application/json;odata=verbose",
        },

        success:function(data)
        {
            dfd.resolve();
            dataReturned(data);
        },
        error:function(data)
        {
            dfd.reject();

        }

    });
  return dfd.promise();
}
function dataReturned(data)
{   
  var k;
  for(k=0;k< data.d.results.length;k++)
  {
     var item=data.d.results[k];
     var subN = item.ImgURL;
     var fileRefTest = item.FileRef;
     console.log(fileRefTest);
  }
}

Answer 1

I have modified you code to replace for loop with recursive call. Check below code try from you side.

var allSiteResults = [];
function getSubmissionNo(siteURL, siteCount) {
    if (siteCount < siteURL.length) {
        siteCount++;
        var currentNewUrl;
        for (a = 0; a <= siteURL.length; a++) {
            currentNewUrl = siteURL[a];
            getDataUsingRest(currentNewUrl).then(function (data) {
                allSiteResults.push(data);
                getSubmissionNo(siteURL, siteCount);
            }).fail(function (error) {
                console.log(error);
            })
        }
    }
    else {
        dataReturned(allSiteResults);
    }
}
function getDataUsingRest(requiredUrl) {
    var dfd = new $.Deferred;
    $.ajax({
        url: requiredUrl + "/_api/web/lists/getbytitle('E-files')/items?    $select=ImgURL,FileRef&$expand=ContentType&$filter=ContentType eq 'Submission Folder'",
        type: "GET",
        asyn: false,
        headers: {
            "Accept": "application/json;odata=verbose",
        },
        success: function (data) {
            dfd.resolve(data);
        },
        error: function (data) {
            dfd.reject(data);
        }
    });
    return dfd.promise();
}
function dataReturned(allSiteResults) {
    $.each(allSiteResults, function (index, data) {
        var k;
        for (k = 0; k < data.d.results.length; k++) {
            var item = data.d.results[k];
            var subN = item.ImgURL;
            var fileRefTest = item.FileRef;
            console.log(fileRefTest);
        }
    })
}
getSubmissionNo(siteURL, 0);