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Writing Actions and Reducers

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What are actions?

Quoting the redux documentation they are:

Actions are payloads of information that send data from your application to your store.

They are simply plain JavaScript objects

/* trigger the panning action of the map to a center point */
const center = [42.3, 36.5];
export const PAN_TO = 'MAP:PAN_TO';
    type: PAN_TO,

They must have type property, typically a constant with a string value, but any other properties are optional

Why we use them

We need them to trigger changes to the application's store via reducers. To do that we use Action Creators

Action Creators

They are simply function that returns actions objects

const defaultValue = [42.3, 36.5];
 * by convention, use an initial name (the action filename)
 * in order to describe better the action type, in this case MAP
 * separated by a colon : and the action constant name
export const PAN_TO = 'MAP:PAN_TO';
export const panTo = (center = defaultValue) => ({
    type: PAN_TO,

Note: Stick to es6 import/export module system and when possible provide a default value for the parameters

These action creators are used in the connected components or in MapStore2 plugins But actions by themselves are not enough we need Reducers that intercepts those actions and change the state accordingly.

Note: Remember to put all the actions .js files in the web/client/actions folder or in js/actions if you are working with custom plugins


Again quoting redux documentation they are:

Reducers specify how the application's state changes in response to actions sent to the store.

Reducers are pure functions that take the previous state and an action and return a new state

(previousState, action) => newState

let's see an example:

// @mapstore is an alias for dir_name/web/client (see webpack.config.js)
import {PAN_TO} from '@mapstore/actions/map';

export default function map(state, action) {
    switch (action.type) {
        case PAN_TO: {
            return {
        default: return state;

As you can see we are changing the center of the map that triggers the panning action of the mapping library

And that's it we have wrote an action and a reducers that make the map panning around.

Note: Remember to put all the reducers .js files in the web/client/reducers folder or in js/reducers if you are working with custom plugins

Advanced usage and tips

Sometimes you need to change a value of an item which is stored inside an array or in a nested object.

Let's imagine we have this object in the store:

layer: {
    features: [object_1, object_2, ...object_n]

And we have created an action that holds the id of the object to change and some properties

export const updateFeature = (id, props = {}) => ({type: UPDATE_LAYER_FEATURE, id, props})

Then in the reducer we can have different implementations. Here we show the one using arrayUpdate from @mapstore/utils/ImmutableUtils for updating objects in array

import {UPDATE_LAYER_FEATURE} from '@mapstore/actions/layer';

export default function layer(state, action) {
    switch (action.type) {
        case UPDATE_LAYER_FEATURE: {
            // lodash find
            const feature = find(state.features, {id:});
            // merge the old and the new feature object, replacing the existing element in the array
            return arrayUpdate("features", {...feature, ...action.props}, {id:}, state);
        default: return state;


Tests in mapstore are stored in __tests__ folder at the same level where actions/reducer are. The file name is the same of the action/reducer with a '-test' suffix


We use expect as testing library, therefore we suggest to have a look there.

How to test an action

Typically you want to test the type and the params return from the action creator

let's test the mapTo action:

// copyright section
import expect from 'expect';
import {panTo, PAN_TO} from '@mapstore/actions/map';
describe('Test correctness of the map actions', () => {
    it('testing panTo', () => {
        const center = [2, 3];
        const returnValue = panTo(center);

In order to speed up the unit test runner, you can: - change the path in build\tests.webpack.js to point to the folder parent of tests for example '../web/client/actions' - then run this command: npm run continuoustest

This allows to run only the tests contained to the specified path. Note: When all tests are successfully passing remember to restore it to its orignal value '../web'

How to test a reducer

Here things can become more complicated depending on your reducer but in general you want to test all cases

// copyright section
import expect from 'expect';
import {panTo} from '@mapstore/actions/map';
import map from '@mapstore/reducers/map';
describe('Test correctness of the map reducers', () => {
    it('testing PAN_TO', () => {
        const center = [2, 3];
        const state = map({}, panTo(center));

        // here you have to check that state has changed accordingly

Here for speedup testing you can again modify the build\tests.webpack.js in order to point to the reducers folder and then runnning npm run continuoustest

Actions and epics

Actions are not only used by redux to update the store (through the reducers), but also for triggering side effects workflows managed by epics

For more details see Writing epics