Where can I find minimum active stake for a certain era?

Question

I'm writing a script using the polkadot.js API. I want to find the minimum active stake for a certain era. I.e. the minimum amount of DOTs staked by a nominator that also got a reward for that era. Where can I find this value or how can I calculate it? Any help appreciated!

EDIT1: I accidentally wrote epoch when I meant era.

Answer 1

What you are asking is basically MinActiveThreshold as defined here.

// the ere in which you are interested.
const currentEra = ...
// mapping from nominators, to the sum backing that they are providing to different validators.
const assignments: Map<string, BN> = new Map();

// use erasStakersClipped if you want to take into account oversubscription as well. 
const stakers = await api.query.staking.erasStakers.entries(currentEra);
stakers.sort((a, b) => a[1].total.toBn().cmp(b[1].total.toBn()))

stakers.map((x) => x[1].others).flat(1).forEach((x) => {
    const nominator = (x as PalletStakingIndividualExposure).who.toString();
    const amount = (x as PalletStakingIndividualExposure).value;
    const val = assignments.get(nominator);
    assignments.set(nominator, val ? amount.toBn().add(val) : amount.toBn())
})

/// transform the map to a flat array..
const nominatorStakes = Array.from(assignments);
// .. and sort it..
nominatorStakes.sort((a, b) => a[1].cmp(b[1]));
// .. where the first item will be minimum rewarded.
const minActiveThreshold = nominatorStakes[0][1];