# 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;
measurements\$.pipe(
scan((acc, curr) => [...acc.slice(-6), curr], [] as measurement[]),
map(measurements => ({
runningDates: measurements.map(({date}) => date),
totalTemp: measurements.reduce((acc,{temp}) => acc + temp, 0),
totalHum: measurements.reduce((acc,{hum}) => acc + hum, 0),
}))
).subscribe(console.log);
```