request_S2_NDVI_for_multiple_dates.js

//VERSION=3
// Script to extract a time series of NDVI values using 
// Sentinel 2 Level 2A data and  metadata file.
function setup() {
  return {
    input: [{
      bands: ["B04", "B08"],
      units: "DN"
    }],
    output: {
      id: "default",
      bands: 1,
      sampleType: SampleType.FLOAT32
    },
    mosaicking: Mosaicking.ORBIT
  }
  
}

// The following function is designed to update the number of
// output bands without knowing beforehand how many there are
function updateOutput(outputs, collection) {
    Object.values(outputs).forEach((output) => {
        output.bands = collection.scenes.length;
    });
}
// function to generate a json file with a list of the NDVI 
// dates used in the analysis. 
function updateOutputMetadata(scenes, inputMetadata, outputMetadata) {
    var dds = [];
    for (i=0; i<scenes.length; i++){
      dds.push(scenes[i].date)
    }
    outputMetadata.userData = { "acquisition_dates":  JSON.stringify(dds) }
}

function evaluatePixel(samples) {
  // Precompute an array to contain NDVI observations
  var n_observations = samples.length;
  let ndvi = new Array(n_observations).fill(0);
  
  // Fill the array with NDVI values
  samples.forEach((sample, index) => {
    ndvi[index] = (sample.B08 - sample.B04) / (sample.B08 + sample.B04) ;
  });
                     
  return ndvi;
}