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@airglow/reducers
A library of simple javascript reducers
Last updated 2 months ago by dgrossen .
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Airglow Reducers

A library for building simple elegant reducers.

Reducer Basics

Take a look at [Redux's Reducer definition] for more details, but basically, a Reducer is a function that transforms a provided state based on an action that takes place.

So basically, you get a function that looks something like this:

(previousState, action) => newState

We pass in the old state and an action and return the new state.

Let's define a couple important rules for Reducers

  1. Reducers must be pure. This means that Reducers do not depend on or modify the state of any variables out of its scope. They are self contained, they never access any data that isn't provided to them
  2. Reducers must not modify the provided state or action. You must clone the state before making any changes. We use libraries like Ramda to modify our objects.

slice

Within our store, we will want to break our state into separate "slices". Look at the following example store:

{
  iceCreamForm: {
    topping: 'chocolate',
    flavor: 'strawberry',
    container: 'waffle cone'
  },
  pizzaForm: {
    topping: 'sausage',
    crust: 'thin',
    size: 'large'
  }
}

We have an ice cream form and a pizza form. Each will likely have a set of unique reducers. The common pattern is to group all of reducers for our slice in a single file using call. call is explained in the next section.

Once we have your separate files, we can create another reducer file that holds our slices:

import { call, slice } from '@airglow/reducers';
import iceCreamReducers from 'iceCream/reducers';
import pizzaReducers from 'pizza/reducers';

export default call(
  slice('iceCreamForm').with(iceCreamReducers),
  slice('pizzaForm').with(pizzaReducers)
);

More info about slices:

  • The reducers of a slice now only see their section of the store. For example:
export const reduceTopping = (state, action) => Rambda.assoc('topping', action.payload, state);

We are provided the iceCreamForm section of the state, and as such, we just need to modify the topping part of that state.

  • Slices can be nested. For example, let's say we now want to have a forms slice in our store. We can do the following:
import { mix, slice } from '@airglow/reducers';
import formReducers from 'form/reducers';

export default slice('form').with(formReducers);

Now we'll have a store that looks like:

form: {
  iceCreamForm: { ... },
  pizzaForm: { ... }
}
  • You can reslice an existing slice. Let's say I'm using the form store in the previous example and I want to add a new form. I can easily do the following:
import { slice } from '@airglow/reducers';
import newFormReducers from 'newForm/reducers';

export default slice('form').with(newFormReducers);

call

The provided call reducer simply calls all it's provided reducers when it is invoked. For example:

import { call } from '@airglow/reducers';
import { toppingReducer, sizeReducer, crustReducer } from 'pizza/reducers';

export default call(
  toppingReducer,
  sizeReducer,
  crustReducer
);

This example is identical to the following:

import { call } from '@airglow/reducers';
import { toppingReducer, sizeReducer, crustReducer } from 'pizza/reducers';

export default (state, action) => {
  let processed = state;
  processed = toppingReducer(state, action);
  processed = sizeReducer(state, action);
  processed = crustReducer(state, action);
  return processed;
);

for

call accepts an addition method that let's you define which actions should trigger the reducer.

export default call(
  call(toppingReducer).for('addTopping', 'removeTopping'),
  call(sizeReducer).for('changeSize'),
  call(crustReducer).for('changeCrust')
);

Here, the topping reducer only gets called when an addTopping or removeTopping action is triggered.

when

call also accepts custom validators. These validators receive the state and action and can test whether or not to run the reducer.

const isNewTopping = (state, action) =>
  state.toppings.indexOf[action.payload] < 0;

export default call(
  call(toppingReducer)
    .for('addTopping')
    .when(isNewTopping)
)

If the topping is already in the list, then we don't run the topping reducer.

reduce

This is a super simple way of saying, always set the value of some field to the value in the payload.

export default call(
  reduce('crust').for('changeCrust')
);

Now if I send in an action like the following:

{ type: changeCrust, payload: 'deep' }

my store's value for crust would update to deep

to

On occassion, you always want to reduce to a specific value. For example:

export default call(
  reduce('mode').to('creating').for('startPizza'),
  reduce('mode').to('buying').for('completePizza'),
);

Now, the mode will always be set to creating for a start pizza event, no matter what's in the payload.

copy

Copies an existing field in the store to another field:

export default call(
  copy('saved').from('pizza').for('savePizza')
);

Now, when we call savePizza, our store will copy the current value in pizza to saved.

extract

Sometimes, your payload is more complex. You can use extract to get the specific fields you care about.

export default call(
  extract('crust', 'size', 'toppings').for('updatePizza')
);

Now, if I send an action like:

{
  type: updatePizza,
  payload:
  {
    crust: 'deep',
    toppings: ['sausage', 'bellpeppers'],
    size: 'small'
  }
}

my store updates accordingly.

Note that the payload keys must match the store keys when using extract.

toggle

Assuming you have a value in your store that is a boolean, toggle can be used to switch back and forth between true and false.

export default call(
  toggle('enabled').for('toggleEnabled')
);

enabled toggles between true and false

initilize

Sets the initial value for an item in the store. This takes an object and a callback. The callback is provided with the default values when the store is empty:

export default initilize(
  {
    crust: 'regular',
    size: 'medium',
    toppings: []
  },
  call(
    extract('crust', 'size', 'toppings').for('updatePizza')
  )
);

Which initilizes the store to:

{
  crust: 'regular',
  size: 'medium',
  toppings: [],
  initializedTo: {
    crust: 'regular',
    size: 'medium',
    toppings: [],
  }
}

Note the initilizedTo section of the store. This holds the initial values so that reset can work properly.

reset

Allows you to reset fields in the store back to their original values. You can either do it one at a time:

export default call(
  initialize('crust').to('regular'),
  initialize('size').to('medium'),
  initialize('toppings').to([]),
  reset('crust', 'size').for('changePizza')
);

Or reset your whole slice at once:

export default call(
  initialize('crust').to('regular'),
  initialize('size').to('medium'),
  initialize('toppings').to([]),
  reset().for('cancelPizza')
);

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