Because -9999 often works better than 0 for a DEM
Follow this link or copy the code below.
// Specify Country names var country_names = ['Indonesia']; // Get the country boundaries var countries = ee.FeatureCollection('ft:1tdSwUL7MVpOauSgRzqVTOwdfy17KDbw-1d9omPw'); // Filter for country var Indonesia = countries.filter(ee.Filter.inList('Country', country_names)); // Get the geometry var Indonesia = Indonesia.geometry(); // Get the bounds var IndonesiaBounds = Indonesia.bounds(); // create the vizualization parameters var viz = {min:0.0, max:1000, palette:"1400f7,00f4e8,f4f000,f40000,960424"}; // add the country boundaries to the map Map.addLayer(Indonesia,null,"Boundaries"); // add the bounds Map.addLayer(IndonesiaBounds,null,"Bounds"); // replace the mask by -9999 var mask = dem.mask() var demMap = ee.Image(-9999).where(mask, dem) // add to map Map.addLayer(demMap.clip(IndonesiaBounds),viz,"Clipped DEM"); //Export the image, specifying scale and region. Export.image.toDrive({ image: demMap, description: 'IndonesiaNoData', scale: 1000, region: IndonesiaBounds });