reduceActionsFrom.md

reduceActionsFrom

Testing helper. Calculates the state after dispatching an sequence of actions to a specific reducer, starting from the a particular state.

// JavaScript

function reduceActionsFrom(state, reducer, ...actions)

// TypeScript

function reduceActionsFrom<S, A extends Action, AS extends A[]>(
  state: S,
  reducer: Reducer<S, A>,
  ...actions: AS
): S

Details

reduceActionsFrom calls reducer with each of the passed actions - as if these had been dispatched in the specified order - and returns the resulting state. For the first reducer call, the given starting state is used; each following call receives the state returned by the previous one.

If called without any actions, reduceActionsFrom simply returns state.

This helper is meant for use in reducer tests. Use this as an alternative to reduceActions if you don't want to start the reduction from the initial state.

Examples

Basic usage:

import { reduceActionsFrom } from 'redux-preboiled'

const reducer = (state = 0, action) => {
  switch (action.type) {
    case 'increment':
      return state + 1
    case 'multiply':
      return state * action.payload
    default:
      return state
  }
}

reduceActionsFrom(0, reducer, { type: 'increment' })
// => 1

reduceActionsFrom(2, reducer, { type: 'increment' })
// => 3

reduceActionsFrom(
  2,
  reducer,
  { type: 'increment' },
  { type: 'increment' },
  { type: 'multiply', payload: 2 }
)
// => 8

reduceActionsFrom(2, reducer)
// => 2

Usage in a Jest test:

import { reduceActionsFrom } from 'redux-preboiled'
import counterReducer, {
  increment,
  multiply
} from './counter.redux'

test('increment', () => {
  const state = reduceActionsFrom(0, counterReducer, increment())
  expect(state).toEqual(1)
})

test('multiply', () => {
  const state = reduceActionsFrom(2, counterReducer, multiply(4))
  expect(state).toEqual(8)
})

test('increment and multiply', () => {
  const state = reduceActionsFrom(
    2,
    counterReducer,
    increment(),
    multiply(4)
  )
  expect(state).toEqual(12)
})

See Also

• reduceActions
• Testing guide