04 Utilities

XSystem exports a few utilities that have proofed themself to be useful while developing the package.

is

A type predicate that narrows an given event to a specific event based in the event type. It is most useful when defining a transition function for Behavior.

import { is } from "xsystem";

type Increment = { type: "increment"; by: number };
type Decrement = { type: "decrement"; by: number };

const event = {} as Increment | Decrement;

if (is<Increment>("increment", event)) {
  // event is narrowed to an `Increment` event.
}

if (is<Decrement>("decrement", event)) {
  // event is narrowed to an `Decrement` event.
}