DCB Example: invoice number sequence

example_01.js

/*
This example shows how the next consecutive integer sequence number (e.g. for creating invoice numbers)

NOTE: In this very basic example, all invoice creation events are iterated. In the actual DCB event store it is possible to only fetch the last event of a given type
*/

const events = [
  {
    type: "INVOICE_CREATED",
    data: { invoiceNumber: 1 },
  },
  {
    type: "INVOICE_CREATED",
    data: { invoiceNumber: 2 },
  },
];

const nextInvoiceNumber = () => {
  const projection = (state, event) => {
    switch (event.type) {
      case "INVOICE_CREATED":
        return event.data.invoiceNumber + 1;
      default:
        return state;
    }
  };
  return events.filter((e) => e.type === "INVOICE_CREATED").reduce(projection, 1);
};

console.log(nextInvoiceNumber()); // 3