Skip to content

How to Map/reduce a series of date bound values to a running total in JavaScript / RXJS?

I have an observable that emits measurement values with a date for a key. Something like:

 { "date" : "2021-11-01",
   "temp" : 23.4,
   "hum"  : 74.5

I need a 7 days running total and average for temp and hum. If I would have a a value for each week, I could write:

const weeklyReducer = (accumulator, currentValue, index) => {
   const key = Math.floor((index-1)/7);
   const workValue = accumulator[key] || {key, temp: 0, hum:0};
   workValue.temp = workValue.temp + currentValue.temp;
   workValue.hum = workValue.hum + currentValue.hum;
   accumulator[key] = workValue;
   return accumulator;

However I need a running total where the values are accumulated like this:

Running total 1: 1
Running total 2: 1,2
Running total 7:  1,2,3,4,5,6,7
Running total 8:  2,3,4,5,6,7,8
Running total 9:  3,4,5,6,7,8,9
Running total 10: 4,5,6,7,8,9,10

How would I design a reducer for this? I’m open for alternative approaches


Something like this?

Here you do re-calculate the total each time. If there were more values or calculating the totals was computationally expensive, you could keep a stack of values and push/pop to subtract old values and push new ones. For a running total of 7, it’s faster to just recalculate with each emission.

I made the observable empty so that this toy example compiles. You’ll need to provide some data instead of an EMPTY stream.

interface measurement {
  date : string,
  temp : number,
  hum  : number

let measurements$: Observable<measurement> = EMPTY;
  scan((acc, curr) => [...acc.slice(-6), curr], [] as measurement[]),
  map(measurements => ({
    runningDates:{date}) => date),
    totalTemp: measurements.reduce((acc,{temp}) => acc + temp, 0),
    totalHum: measurements.reduce((acc,{hum}) => acc + hum, 0),