time series - Getting weekly mean composite in Google Earth Engine?

I am currently working on a time-series in Google Earth Engine. Iam still quite inexperienced, especially when it comes to functions. I want to extract the mean values of Sentinel1-images for each week in a period of 7 month and aggregate them into a new Image Collection I can then further process.

So far I tried to do it for a single month:

 var testsubset = ee.FeatureCollection("users/xyz/test_subset");

//set start-dates    

var images = {
  '2018-02-01': getWeeklySentinelComposite('2018-02-01'),
  '2018-02-08': getWeeklySentinelComposite('2018-02-08'),

  '2018-02-15': getWeeklySentinelComposite('2018-02-15'),
  '2018-02-22': getWeeklySentinelComposite('2018-02-22'),
  '2018-03-01': getWeeklySentinelComposite('2018-03-01')
};

//function to get a weekly composite

function getWeeklySentinelComposite(date) {
  var date = ee.Date(date);
  // Only include the VV polarization, for consistent compositing.

  var polarization = 'VV';
  var sentinel1 = ee.ImageCollection('COPERNICUS/S1_GRD')
                      .filterBounds(testsubset)
                      .filterDate(date, date.advance(1, 'week'))
                      .filter(ee.Filter.listContains('transmitterReceiverPolarisation', polarization))
                      .filter(ee.Filter.eq('instrumentMode', 'IW'))
                      .select(polarization)
                      .mean();
  return sentinel1;
}

I currently run into two main issues: I'am not sure how to return the images I picked and how to turn them into a new ImageCollection. Second: I get

"Invalid argument specified for ee.Date(): [object Object]"

and don't really know why (I used a similar function for another script and it worked fine)

Just to clarify, this is what I want to do with the Image Collection in the end:

//Map.addLayer(testsubset, {color:'green'}, 'Testshape', true , 0.5);

var tempTimeSeries = ui.Chart.image.series({
  imageCollection: sentinel1_IC,
  region: testsubset,

  reducer: ee.Reducer.mean(),
  scale: 200,
  xProperty: 'system:time_start'
});

tempTimeSeries.setChartType('ScatterChart');
tempTimeSeries.setOptions({
  title: 'VV-over-time',
  vAxis: {
    title: 'sigma VV'

  },
  lineWidth: 1,
  pointSize: 4,
  series: {
    0: {color: 'blue'},
    1: {color: 'red'},
    2: {color: 'green'}
  }
});


print(tempTimeSeries);

Answer

You are almost there. The fact you cannot get an imagecollection from your function now, is that you make a clientside dictionary with dates as key. Instead, first make a server side list of dates seperated a week apart:

//set a list of start dates
var startDate = '2018-02-01';
var endDate = '2018-12-31';
var weekDifference = ee.Date(startDate).advance(1, 'week').millis().subtract(ee.Date(startDate).millis());
var listMap = ee.List.sequence(ee.Date(startDate).millis(), ee.Date(endDate).millis(), weekDifference);

Once you have this, you can map over the list and return an image for every start date of a week with your function. Note that you will have to set something like a date property to every image to work with them later on. You can transform this list of image to an image collection using ee.ImageCollection.fromImages():

var sentinel1_IC = ee.ImageCollection.fromImages(listMap.map(function(dateMillis){
  var date = ee.Date(dateMillis);
  return getWeeklySentinelComposite(date);
}));

You can then continue with your analysis. Link script