Reducing event streams

Here is a simple event stream:

const events = [
    { type: 'account opened', credit: 5000 },
    { type: 'cheque deposited', credit: 50000 },
    { type: 'bill paid', debit: 10000, payee: 'AT&T' }

This event stream is located in memory but let’s ignore that for now.

We’d like to know what the current account balance is, given above transactions.

const rules = (account, event) => {
    if (event.type === 'account opened') {
        account.balance =
    else if (event.type === 'cheque deposited') {
        account.balance +=
    else if (event.type === 'bill paid') {
        account.balance -= event.debit
    return account

const account = events.reduce(rules, {})


Prints out 45000 which is exactly what you’d expect: 5000 + 50000 - 10000 = 45000

Run it on JS Bin

Since this is a toy example, it is missing a number of important features:

  • Data is not saved anywhere

  • Retrieving the current balance requires looping over all events in the stream

  • There is no type checking performed

  • There is no mechanism for other components to be notified when a new event is saved

These are all issues that Persistr solves.

Here’s what the above code looks like using Persistr. It’s a one-liner to retrieve the current value of a domain object:

const account = await stream.object('Bank Account').latest()

Persistr is about making event-sourcing simple.

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