Heap-based online algorithm for median calculation. To find median in unsorted stream of data, two heaps are created. Max-heap stores values less then M, min-heap - more than M. Each time new input value arrives, the algorithm rebalances heaps if needed and then adds value to one of them.
If you have a really large data stream, it'd be better to use a probabilistic lossy algorithm.
npm i -S online-median
const Median = require('online-median')
const median = Median()
;[1,2,3,4,5,6].forEach(v => median(v))
console.log(median()) // -> 3.5