setDefaultOptions

// Set global locale:
import { es } from 'date-fns/locale'
setDefaultOptions({ locale: es })
const result = format(new Date(2014, 8, 2), 'PPPP')
//=> 'martes, 2 de septiembre de 2014'

// Start of the week for 2 September 2014:
const result = startOfWeek(new Date(2014, 8, 2))
//=> Sun Aug 31 2014 00:00:00

// Start of the week for 2 September 2014,
// when we set that week starts on Monday by default:
setDefaultOptions({ weekStartsOn: 1 })
const result = startOfWeek(new Date(2014, 8, 2))
//=> Mon Sep 01 2014 00:00:00

// Manually set options take priority over default options:
setDefaultOptions({ weekStartsOn: 1 })
const result = startOfWeek(new Date(2014, 8, 2), { weekStartsOn: 0 })
//=> Sun Aug 31 2014 00:00:00

// Remove the option by setting it to `undefined`:
setDefaultOptions({ weekStartsOn: 1 })
setDefaultOptions({ weekStartsOn: undefined })
const result = startOfWeek(new Date(2014, 8, 2))
//=> Sun Aug 31 2014 00:00:00