First, we need some actions. We'll want to increment and decrement the counter, and we'll also want to reset it to zero. The first two requirements can be achieved with a single action (decrementing is just incrementing by a negative amount), so we'll need two actions, each identified by a constant:

// Source file: src/counterApp/counter.actions.js

/* @flow */

export const COUNTER_INCREMENT = "counter:increment";
export const COUNTER_RESET = "counter:reset";

export type CounterAction = {
    type: string,
    value?: number
};

export const reset = () =>
    ({
        type: COUNTER_RESET
    }: CounterAction);

export const increment = (inc: number) =>
    ({
        type: COUNTER_INCREMENT,
        value: inc
    }: CounterAction);

export const decrement = (dec: number) =>
    ({
        type: COUNTER_INCREMENT,
        value: -dec
    }: CounterAction);

// returning increment(-dec) would have worked as well

In fact, we should say that increment(), decrement(), and reset() are action creators; the actual actions are the values returned by those functions.