feat(settlement): create types representing the result of a promise

src/types.test.js

@@ -0,0 +1,38 @@
+/** @format */
+
+import { Fulfillment, Rejection } from "./types";
+import _ from "lodash";
+
+describe("Fulfillment", () => {
+	_.forEach([true, { foo: true }], (arg) => {
+		describe(`when passed the ${typeof arg} argument '${JSON.stringify(
+			arg
+		)}'`, () => {
+			const fulfillment = new Fulfillment(arg);
+
+			it("exposes the argument on the 'value' property", () => {
+				expect(fulfillment).toHaveProperty("value", arg);
+			});
+
+			it("has a 'status' property of 'fulfilled'", () => {
+				expect(fulfillment).toHaveProperty("status", "fulfilled");
+			});
+		});
+	});
+});
+
+describe("Rejection", () => {
+	_.forEach(["BOOM!", new Error("BOOM!")], (arg) => {
+		describe(`when passed the ${typeof arg} argument: '${arg}'`, () => {
+			const rejection = new Rejection(arg);
+
+			it("returns the argument in the 'reason' property", () => {
+				expect(rejection).toHaveProperty("reason", arg);
+			});
+
+			it("has a 'status' property whose value is 'rejected'", () => {
+				expect(rejection).toHaveProperty("status", "rejected");
+			});
+		});
+	});
+});

src/types.ts

@@ -59,3 +59,30 @@ export type PromisableIterable<T> = Promisable<IterableOfPromisables<T>>;
  * although it may produce either values or promises of values or both.
  */
 export type IterableOfPromisables<T> = Iterable<Promisable<T>>;
+
+/**
+ * Represents a `PromiseSettledResult` value for a fulfilled promise.
+ */
+export class Fulfillment<T> implements PromiseFulfilledResult<T> {
+	constructor(public readonly value: T) {}
+
+	get status(): "fulfilled" {
+		return "fulfilled";
+	}
+}
+
+/**
+ * Represents a `PromiseSettledResult` value for a rejected promise.
+ */
+export class Rejection implements PromiseRejectedResult {
+	constructor(public readonly reason: unknown) {}
+
+	get status(): "rejected" {
+		return "rejected";
+	}
+}
+
+/**
+ * Equivalent to a `PromiseSettledResult`, but specific to our classes.
+ */
+export type Settlement<T> = Fulfillment<T> | Rejection;