Google Earth Engine: reprojecting over an image collection

Question

Mapping over an image collection (NO2 in this case), for .clip, is simple:

var NO2NL=NO2.map(function(im){ 
   return im.clip(NL);
});

Replacing .clip with .resample(bilinear) works too, however using .reproject(scale=1000) always produces an error: Variable must be declared with "var": scale. Any idea how I can reproject over an image collection?

Entire script for reference:

//Set variables

var countries = ee.FeatureCollection("FAO/GAUL/2015/level0");
var NL = countries.filterMetadata('ADM0_NAME', 'equals', 'Netherlands');
var start_period = ee.Date('2018-07-01');
var end_period = ee.Date(new Date().getTime());

var NO2 = ee.ImageCollection('COPERNICUS/S5P/OFFL/L3_NO2')
    .select('tropospheric_NO2_column_number_density')
    .filterDate(start_period, end_period);
  
var NO2NL=NO2.map(function(im){ 
   return im.clip(NL);
});


//+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

// Date slider parameters
ee.Dictionary({start: start_period, end: end_period})
  .evaluate(renderSlider);

function renderSlider(dates) {
  var slider = ui.DateSlider({
    start: dates.start.value, 
    end: dates.end.value, 
    period: 14, // Every 14 days
    onChange: renderDateRange,
    style: {
    position: 'top-left',
  }
  });
  Map.add(slider);
}

function renderDateRange(dateRange) {
  var image = NO2NL
    .filterDate(dateRange.start(), dateRange.end())
    .median();

//+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
  
 // Visual parameters and mapping
  var vis = {min: 0, max: 0.0001, palette: [
    'black', 'blue', 'purple', 'cyan', 'green', 'yellow', 'red']};
  var layer = ui.Map.Layer(image, vis, 'Tropospheric NO2');
  Map.layers().reset([layer]);
}
Map.setCenter(5.5, 52.2, 7.5);

Eventually I hope that the time slider shows only dates on which the data is available. I also want to use ECMWF data for wind speed and join the datasets so that only days with wind speed <10kph are shown!

Answer 1

You need to also set crs. Then in Earth Engine javascript you need to define all arguments up to the one you want to change, or use a dictionary. So this would work:

return im.reproject({crs:'EPSG:4326', scale:1000});

as well as this:

return im.reproject('EPSG:4326', null, 1000);