If you know the bucket that you're working on you can call ListObjects or ListObjectsv2 on that bucket to get up to 1000 objects from it.

http://docs.aws.amazon.com/AWSJavaScriptSDK/latest/AWS/S3.html#listObjects-property

var params = {
  Bucket: 'STRING_VALUE', /* required */
  Delimiter: 'STRING_VALUE',
  EncodingType: url,
  Marker: 'STRING_VALUE',
  MaxKeys: 0,
  Prefix: 'STRING_VALUE',
  RequestPayer: requester
};
s3.listObjects(params, function(err, data) {
  if (err) console.log(err, err.stack); // an error occurred
  else     console.log(data);           // successful response
  // put code here to filter out just the .zips, then you can request those files with a GetObject() request
});

You can call your lambda function manually whenever you want to get information about new files. With this code you will always get the last file modified / created.

s3.listObjects(params, function (err, data) {
if (err)
    console.log(err, err.stack); // an error occurred

var sortArray;

data.Contents.sort(function(a,b) {
    return (b.LastModified > a.LastModified) ? 1 :
    ((a.LastModified > b.LastModified) ? -1 : 0);
});

for(var file of data.Contents){
    if (file.Key.endsWith('.zip')) {
        //extractData(file.Key);
        break;
    }
}

But we can have a problem like this, if there is no new file created, it will happen to extract the same file more than once. I suggest later that using the file delete or find another way to identify that file has already been used.

I hope it helped you!