declare namespace I18n {
  type Scope = string | string[]

  let defaultLocale: string
  let locale: string
  let defaultSeparator: string
  let placeholder: RegExp
  let fallbacks: boolean
  let missingBehaviour: 'message' | 'guess'
  let missingTranslationPrefix: string

  // tslint:disable-next-line prefer-declare-function
  let missingTranslation: (
    scope: string,
    options?: TranslateOptions
  ) => string | null | undefined
  // tslint:disable-next-line prefer-declare-function
  let missingPlaceholder: (
    placeholder: string,
    message: string,
    options?: InterpolateOptions
  ) => string | null | undefined
  // tslint:disable-next-line prefer-declare-function
  let nullPlaceholder: (
    placeholder: string,
    message: string,
    options?: InterpolateOptions
  ) => string | null | undefined

  let translations: { [locale: string]: object }
  let locales: {
    [key: string]: string | string[] | ((locale: string) => string | string[])
  }
  let pluralization: { [locale: string]: (count: number) => string[] }

  function reset(): void

  function currentLocale(): string

  interface InterpolateOptions {
    [key: string]: any // interpolation
  }

  interface TranslateOptions extends InterpolateOptions {
    scope?: Scope
    message?: string
    defaults?: Array<{ message: string } | { scope: Scope }>
    defaultValue?: string
  }
  function translate(scope: Scope, options?: TranslateOptions): string
  function t(scope: Scope, options?: TranslateOptions): string

  function localize(
    scope: 'currency' | 'number' | 'percentage',
    value: number,
    options?: InterpolateOptions
  ): string
  function localize(
    scope: Scope,
    value: string | number | Date,
    options?: InterpolateOptions
  ): string
  function l(
    scope: 'currency' | 'number' | 'percentage',
    value: number,
    options?: InterpolateOptions
  ): string
  function l(
    scope: Scope,
    value: string | number | Date,
    options?: InterpolateOptions
  ): string

  interface ToNumberOptions {
    precision?: number
    separator?: string
    delimiter?: string
    strip_insignificant_zeros?: boolean
  }
  function toNumber(num: number, options?: ToNumberOptions): string

  type ToPercentageOptions = ToNumberOptions
  function toPercentage(num: number, options?: ToPercentageOptions): string

  interface ToCurrencyOptions extends ToNumberOptions {
    format?: string
    unit?: string
    sign_first?: boolean
  }
  function toCurrency(num: number, options?: ToCurrencyOptions): string

  function toTime(scope: Scope, value: string | number | Date): string

  interface ToHumanSizeOptions extends ToNumberOptions {
    format?: string
  }
  function toHumanSize(num: number, options?: ToHumanSizeOptions): string

  function strftime(date: Date, format: string): string
}

declare module '@procore/core-i18n-js' {
  type VendorI18n = typeof I18n

  export const locale = {
    en: 'en',
    enCA: 'en-CA',
    frCA: 'fr-CA',
    es: 'es',
    AU: 'en-AU',
    enGB: 'en-GB',
    ko: 'ko',
    th: 'th-TH',
    pt: 'pt',
    is: 'is-IS',
    pseudo: 'pseudo',
  } as const

  export type Locale = typeof locale[keyof typeof locale]

  interface I18njs extends VendorI18n {
    locale: Locale
  }

  interface I18njsConfig {
    I18n?: I18njs
    locale?: Locale
    translations?: {}
  }

  function createI18njs(config: I18njsConfig): I18njs
}