I cannot export all image collection with image date for MODIS NDVI

Question

While exporting images the namings are not setting properly with the image date. How can I solve the problem?

I have attached the associated code:

// Import the MODIS NDVI dataset.
var modisNDVI = ee.ImageCollection("MODIS/006/MOD13Q1")
  .select("NDVI"); // Select the NDVI band.

// Define a region of interest (ROI) or multiple ROIs (e.g., a feature collection).
var roi = ee.Geometry.Rectangle(-120.0, 35.0, -70.0, 45.0); // Example ROI.

// Define a time period.
var startDate = ee.Date('2021-01-01');
var endDate = ee.Date('2021-12-31');

// Function to export all MODIS NDVI images in the collection.
var exportAllNDVI = function(collection, roi, startDate, endDate) {
  var imageList = collection
    .filterDate(startDate, endDate)
    .filterBounds(roi)
    .toList(collection.size());
  
  var imageCount = imageList.size().getInfo();
  
  for (var i = 0; i < imageCount; i++) {
    var image = ee.Image(imageList.get(i));
    var date = ee.Date(image.get('system:time_start')).format("YYYY-MM-dd");
    
    // Export the image to Google Drive with the date as the filename.
    Export.image.toDrive({
      image: image,
      description: 'MODIS_NDVI_' + date,
      folder: 'MODIS_NDVI_Exports',
      region: roi,
      scale: 250, // Adjust the scale as needed.
      crs: 'EPSG:4326',
      maxPixels: 1e13,
      fileFormat: 'GeoTIFF'
    });
  }
};

// Run the exportAllNDVI function for the specified ROI and time period.
exportAllNDVI(modisNDVI, roi, startDate, endDate);

Answer 1

The problem is that date is a server-side ee.String while the exported description must be a client-side string. To resolve this, you will need to use getInfo or evaluate to retrieve the formatted date string before exporting.

Below is a modified version of exportAllNDVI that uses evaluate to asynchronously retrieve each image date. Once the date is available, the callback function will start the export.

var exportAllNDVI = function(collection, roi, startDate, endDate) {
  var imageList = collection
    .filterDate(startDate, endDate)
    .filterBounds(roi)
    .toList(collection.size());
  
  var imageCount = imageList.size().getInfo();
  
  for (var i = 0; i < imageCount; i++) {
    var image = ee.Image(imageList.get(i));
    var date = ee.Date(image.get('system:time_start')).format("YYYY-MM-dd");
    
    date.evaluate(function(date_string) {
      // Export the image to Google Drive with the date as the filename.
      Export.image.toDrive({
        image: image,
        description: 'MODIS_NDVI_' + date_string,
        folder: 'MODIS_NDVI_Exports',
        region: roi,
        scale: 250, // Adjust the scale as needed.
        crs: 'EPSG:4326',
        maxPixels: 1e13,
        fileFormat: 'GeoTIFF'
      });
    });
  }
};