Conditionally Map.addLayer() in Google Earth Engine

Question

My app has a dateSlider that searches multiple collections, and selects the image nearest to a date within a window. I want to conditionally display those images, with error handling for empty images.

var image = l8sr
    .filterDate('2016-01-01', '2016-07-01')
    .first()

var nullImage = s2sr
  .filterDate('2016-01-01', '2016-07-01') // returns empty collection
  .first()
print(nullImage) // null



Map.addLayer(nullImage) // Error! Expression does not evaluate to an image.

// Check before trying to add the image to map  
if(nullImage !== null){Map.addLayer(nullImage)} // Same error

// Dosen't work server-side either
ee.Algorithms.If(nullImage !== null, Map.addLayer(nullImage)) // Same error

I want my script to check each image before it tries to add a layer, to avoid the error message. In other words, I want my script to be robust to handle empty image collections, so it can flexibly add layers to the map. How can I accomplish this?

https://code.earthengine.google.com/0502525f29a25d7f1d4ab7b63acde203

Answer 1

Map.addLayer is client-side. Whether the image computation evaluates to null is server-side. So you need to bridge the gap by asking for the value to be computed:

image.evaluate(function (imageInfo) {
  if (imageInfo) {
    Map.addLayer(image);
  }
});

Note that this will also do nothing if there was an error computing the image collection for any reason; to be able to see if that's the case you need to check for an error using the second callback parameter:

image.evaluate(function (imageInfo, error) {
  if (imageInfo) {
    Map.addLayer(image);
  } else if (error) {
    print("Error:", error);
  }
});