actions.js

const noop = () => {}

function formatProductForRequest(product) {
  // ...
}

export function addToCart(product, onSuccess = noop, onFailure = noop) {
  return (dispatch, getState) => {
    dispatch(cartAddAttempt(())

    const params = formatProductForRequest(product)
    fetch(params)
      .then((res) => dispatch(cartAddSuccess(res)))
      .then(onSuccess)
      .catch((res) => dispatch(cartAddFailure(res)))
      .then(onFailure)
  }
}

@scottluptowski
Minor modifications:
cartAddSuccess needs to be onSuccess and cartAddFailure => onFailure