Writing tests for epics with state$

epicTestFunction.ts

import { Action } from "redux";
import { ActionsObservable, Epic, StateObservable } from "redux-observable";
import { Subject } from "rxjs";
import { toArray } from "rxjs/operators";

const stateInput$ = new Subject();
const state$ = new StateObservable<any>(stateInput$, undefined);

interface IEpicTest<A extends Action, S, D> {
    (params: {
        epic: Epic<A, S, D>;
        inputActions: A[];
        expectedActions?: A[];
        state?: S;
        dependencies?: D;
    }): Promise<A[]>;
}

export const epicTest: IEpicTest<Action, {}, {}> = ({
    epic,
    inputActions,
    expectedActions,
    state = {},
    dependencies = {},
}) => {
    stateInput$.next(state);

    const action$ = ActionsObservable.from(inputActions);

    return epic(action$, state$, dependencies)
        .pipe(toArray())
        .toPromise()
        .then(actualActions => {
            if (expectedActions !== undefined) {
                expect(actualActions).toEqual(expectedActions);
            }
            return actualActions;
        });
};

/* then in your actual test file */
import { epicTest } from "./epicTestFunction.ts";
import { myEpic } from "./myEpic.ts";
    
describe("myEpic", () => {
    it("should asdf", () => epicTest({
        epic: myEpic,
        inputActions: [/* actions */],
        expectedActions: [/* expected actions */],
        state: {/* state */},
    }).then(output => {
        // do something extra like take snapshots
    }));
});