Exporting several (6 tiles) Landsat NDVI Google Earth Engine output from median images to my drive

Question

I am new to Google Earth Engine. I managed to produce NDVI on the map window from 6 Landsat tiles from median values .median(). With the .first() I managed to export it but with the median() I get several tif files without data. What is the problem? Here is the code I used.

// var surfaceReflectanceL5 = ee.ImageCollection('LANDSAT/LT05/C01/T1_SR');
var img = ee.Image(ee.ImageCollection('LANDSAT/LC08/C01/T1_SR')
    .filterBounds(roi)
    .filterDate('2018-01-01', '2018-12-31')
    .sort('CLOUD_COVER')
    .median());
Map.addLayer(img, {bands: ['B5', 'B4','B3'],min:0, max: 3000}, 'True colour image');


// Use the normalizedDifference(A, B) to compute (A - B) / (A + B)
var ndvi = img.normalizedDifference(['B5', 'B4']);

// Make a palette: a list of hex strings.
var palette = ['FFFFFF', 'CE7E45', 'DF923D', 'F1B555', 'FCD163', '99B718',
               '74A901', '66A000', '529400', '3E8601', '207401', '056201',
               '004C00', '023B01', '012E01', '011D01', '011301'];

// Display the input image and the NDVI derived from it.
Map.addLayer(img.select(['B5', 'B4', 'B3']),
         {gain: [0.1, 0.1, 0.1]});
Map.addLayer(ndvi, {min: 0, max: 1, palette: palette}, 'NDVI');

// the problem is here: why it is exporting several layers without data (sizes less than 200kb)
// Export the image, specifying scale and region.
Export.image.toDrive({
  image: ndvi,
 description: 'NDVI_2018_A1',
 scale: 30,
 region: roi
 });

Answer 1

I am not exactly sure on what it does but after I used the reduce command, I managed to export my several NDVI tiles all at once with the NDVI scale I wanted. Hope someone with good skill will comment/confirm on this.

// reduce
var reducedNDVI = ndvi.reduce(ee.Reducer.median())

// Export the image, specifying scale and region.

Export.image.toDrive({
     image: reducedNDVI,
     description: 'NDVI_2018_All',
     scale: 30,
     region: roi
 });