diff --git a/.eslintrc b/.eslintrc
index 729d68e..043b9a9 100644
--- a/.eslintrc
+++ b/.eslintrc
@@ -6,5 +6,8 @@
   },
   "parserOptions": {
     "sourceType": "module"
+  },
+  "rules": {
+    "@typescript-eslint/camelcase": "off"
   }
 }
diff --git a/src/client/includes/formatters.ts b/src/client/includes/formatters.ts
index 0a8a40d..abe1cb9 100644
--- a/src/client/includes/formatters.ts
+++ b/src/client/includes/formatters.ts
@@ -3,47 +3,52 @@ import memoize from 'fast-memoize'
 
 import { MemoizedIntlFormatter } from '../types'
 import { getCurrentLocale } from '../stores/locale'
-import { getFormats } from '../configs'
+import { getOptions } from '../configs'
 
 const getIntlFormatterOptions = (
   type: 'time' | 'number' | 'date',
   name: string
 ): any => {
-  const formats = getFormats()
+  const formats = getOptions().formats
   if (type in formats && name in formats[type]) {
     return formats[type][name]
   }
 
-  if (
-    type in IntlMessageFormat.formats &&
-    name in IntlMessageFormat.formats[type]
-  ) {
-    return (IntlMessageFormat.formats[type] as any)[name]
-  }
-
-  return null
+  throw new Error(`[svelte-i18n] Unknown "${name}" ${type} format.`)
 }
 
 export const getNumberFormatter: MemoizedIntlFormatter<
   Intl.NumberFormat,
   Intl.NumberFormatOptions
-> = memoize((options = {}) => {
-  const locale = options.locale || getCurrentLocale()
-  if (options.format) {
-    const format = getIntlFormatterOptions('number', options.format)
-    if (format) options = format
+> = memoize(({ locale, format, ...options } = {}) => {
+  locale = locale || getCurrentLocale()
+  if (locale == null) {
+    throw new Error('[svelte-i18n] A "locale" must be set to format numbers')
   }
+
+  if (format) {
+    options = getIntlFormatterOptions('number', format) || {}
+  }
+
   return new Intl.NumberFormat(locale, options)
 })
 
 export const getDateFormatter: MemoizedIntlFormatter<
   Intl.DateTimeFormat,
   Intl.DateTimeFormatOptions
-> = memoize((options = { format: 'short' }) => {
-  const locale = options.locale || getCurrentLocale()
+> = memoize(({ locale, format, ...options } = {}) => {
+  locale = locale || getCurrentLocale()
+  if (locale == null) {
+    throw new Error('[svelte-i18n] A "locale" must be set to format dates')
+  }
 
-  const format = getIntlFormatterOptions('date', options.format)
-  if (format) options = format
+  const hasInlineArgs = Object.keys(options).length > 0
+  if (!hasInlineArgs) {
+    options =
+      typeof format === 'string'
+        ? getIntlFormatterOptions('date', format)
+        : getIntlFormatterOptions('date', 'short')
+  }
 
   return new Intl.DateTimeFormat(locale, options)
 })
@@ -51,16 +56,26 @@ export const getDateFormatter: MemoizedIntlFormatter<
 export const getTimeFormatter: MemoizedIntlFormatter<
   Intl.DateTimeFormat,
   Intl.DateTimeFormatOptions
-> = memoize((options = { format: 'short' }) => {
-  const locale = options.locale || getCurrentLocale()
+> = memoize(({ locale, format, ...options } = {}) => {
+  locale = locale || getCurrentLocale()
+  if (locale == null) {
+    throw new Error(
+      '[svelte-i18n] A "locale" must be set to format time values'
+    )
+  }
 
-  const format = getIntlFormatterOptions('time', options.format)
-  if (format) options = format
+  const hasInlineArgs = Object.keys(options).length > 0
+  if (!hasInlineArgs) {
+    options =
+      typeof format === 'string'
+        ? getIntlFormatterOptions('time', format)
+        : getIntlFormatterOptions('time', 'short')
+  }
 
   return new Intl.DateTimeFormat(locale, options)
 })
 
 export const getMessageFormatter = memoize(
   (message: string, locale: string) =>
-    new IntlMessageFormat(message, locale, getFormats())
+    new IntlMessageFormat(message, locale, getOptions().formats)
 )
diff --git a/src/client/types/index.ts b/src/client/types/index.ts
index 15fdcd3..8cc4651 100644
--- a/src/client/types/index.ts
+++ b/src/client/types/index.ts
@@ -22,7 +22,7 @@ type IntlFormatterOptions<T> = T & {
 }
 
 export interface MemoizedIntlFormatter<T, U> {
-  (options: IntlFormatterOptions<U>): T
+  (options?: IntlFormatterOptions<U>): T
 }
 
 export interface Formatter extends FormatterFn {
diff --git a/test/client/formatter.test.ts b/test/client/formatter.test.ts
new file mode 100644
index 0000000..426d350
--- /dev/null
+++ b/test/client/formatter.test.ts
@@ -0,0 +1,154 @@
+import {
+  getNumberFormatter,
+  getDateFormatter,
+  getTimeFormatter,
+  getMessageFormatter,
+} from '../../src/client/includes/formatters'
+import { init } from '../../src/client/configs'
+
+beforeEach(() => {
+  init({ fallbackLocale: undefined })
+})
+
+describe('number formatter', () => {
+  const number = 123123
+  test('should format a date according to the current locale', () => {
+    init({ fallbackLocale: 'en' })
+    expect(getNumberFormatter().format(number)).toBe('123,123')
+  })
+
+  test('should format a number according to a locale', () => {
+    init({ fallbackLocale: 'en' })
+    expect(getNumberFormatter({ locale: 'pt-BR' }).format(number)).toBe(
+      '123.123'
+    )
+  })
+
+  test('should format a number with a custom format', () => {
+    init({
+      fallbackLocale: 'en',
+      formats: require('../fixtures/formats.json'),
+    })
+
+    expect(getNumberFormatter({ format: 'brl' }).format(number)).toBe(
+      'R$123,123.00'
+    )
+  })
+
+  test('should format a number with inline options', () => {
+    init({ fallbackLocale: 'en' })
+
+    expect(
+      getNumberFormatter({ style: 'currency', currency: 'BRL' }).format(number)
+    ).toBe('R$123,123.00')
+  })
+})
+
+describe('date formatter', () => {
+  const date = new Date(2019, 1, 1)
+
+  test('should format a date according to the current locale', () => {
+    init({ fallbackLocale: 'en' })
+    expect(getDateFormatter().format(date)).toBe('2/1/19')
+  })
+
+  test('should format a date according to a locale', () => {
+    expect(getDateFormatter({ locale: 'pt-BR' }).format(date)).toBe('01/02/19')
+  })
+
+  test('should throw if passed a non-existing format', () => {
+    init({
+      fallbackLocale: 'en',
+      formats: require('../fixtures/formats.json'),
+    })
+
+    expect(() =>
+      getDateFormatter({ locale: 'pt-BR', format: 'foo' }).format(date)
+    ).toThrowError(`[svelte-i18n] Unknown "foo" date format.`)
+  })
+
+  test('should format a date with a custom format', () => {
+    init({
+      fallbackLocale: 'en',
+      formats: require('../fixtures/formats.json'),
+    })
+
+    expect(getDateFormatter({ format: 'customDate' }).format(date)).toBe(
+      '2019 AD'
+    )
+  })
+
+  test('should format a date with inline options', () => {
+    init({ fallbackLocale: 'en' })
+
+    expect(
+      getDateFormatter({ year: 'numeric', era: 'short' }).format(date)
+    ).toBe('2019 AD')
+  })
+})
+
+describe('time formatter', () => {
+  const time = new Date(2019, 1, 1, 20, 37, 32)
+
+  test('should format a time according to the current locale', () => {
+    init({ fallbackLocale: 'en' })
+    expect(getTimeFormatter().format(time)).toBe('8:37 PM')
+  })
+
+  test('should format a time according to a locale', () => {
+    expect(getTimeFormatter({ locale: 'pt-BR' }).format(time)).toBe('20:37')
+  })
+
+  test('should format a time with a custom format', () => {
+    init({
+      fallbackLocale: 'en',
+      formats: require('../fixtures/formats.json'),
+    })
+
+    expect(getTimeFormatter({ format: 'customTime' }).format(time)).toBe(
+      '08:37:32 PM'
+    )
+  })
+
+  test('should throw if passed a non-existing format', () => {
+    init({
+      fallbackLocale: 'en',
+      formats: require('../fixtures/formats.json'),
+    })
+
+    expect(() =>
+      getTimeFormatter({ locale: 'pt-BR', format: 'foo' }).format(time)
+    ).toThrowError(`[svelte-i18n] Unknown "foo" time format.`)
+  })
+
+  test('should format a time with inline options', () => {
+    init({ fallbackLocale: 'en' })
+
+    expect(
+      getTimeFormatter({
+        hour: '2-digit',
+        minute: '2-digit',
+        second: '2-digit',
+      }).format(time)
+    ).toBe('08:37:32 PM')
+  })
+})
+
+describe('message formatter', () => {
+  test('formats a message with interpolated values', () => {
+    expect(
+      getMessageFormatter('Page: {current,number}/{max,number}', 'en').format({
+        current: 2,
+        max: 10,
+      })
+    ).toBe('Page: 2/10')
+  })
+
+  test('formats number with custom formats', () => {
+    expect(
+      getMessageFormatter('Number: {n, number, compactShort}', 'en').format({
+        n: 2000000,
+      })
+    ).toBe('Number: 2M')
+  })
+})
diff --git a/test/client/index.test.ts b/test/client/index.test.ts
deleted file mode 100644
index 485c311..0000000
--- a/test/client/index.test.ts
+++ /dev/null
@@ -1,172 +0,0 @@
-// // TODO remake this, it's a mess
-// import { Formatter } from '../../src/client/types'
-// import {
-//   dictionary,
-//   locale,
-//   format,
-//   addCustomFormats,
-//   customFormats,
-//   register,
-//   waitLocale,
-// } from '../../src/client'
-// import { getClientLocale } from '../../src/client/includes/utils'
-
-// global.Intl = require('intl')
-
-// let _: Formatter
-// let currentLocale: string
-
-// const dict = {
-//   en: require('../fixtures/en.json'),
-// }
-
-// register('en-GB', () => import('../fixtures/en-GB.json'))
-// register('pt', () => import('../fixtures/pt.json'))
-// register('pt-BR', () => import('../fixtures/pt-BR.json'))
-// register('pt-PT', () => import('../fixtures/pt-PT.json'))
-
-// format.subscribe(formatFn => {
-//   _ = formatFn
-// })
-// dictionary.set(dict)
-// locale.subscribe((l: string) => {
-//   currentLocale = l
-// })
-
-// describe('locale', () => {
-//   it('should change locale', async () => {
-//     await locale.set('en')
-//     expect(currentLocale).toBe('en')
-
-//     await locale.set('en-US')
-//     expect(currentLocale).toBe('en-US')
-//   })
-// })
-
-// describe('dictionary', () => {
-//   it('load a partial dictionary and merge it with the existing one', async () => {
-//     await locale.set('en')
-//     register('en', () => import('../fixtures/partials/en.json'))
-//     expect(_('page.title_about')).toBe('page.title_about')
-
-//     await waitLocale('en')
-//     expect(_('page.title_about')).toBe('About')
-//   })
-// })
-
-// describe('formatting', () => {
-//   it('should translate to current locale', async () => {
-//     await locale.set('en')
-//     expect(_('switch.lang')).toBe('Switch language')
-//   })
-
-//   it('should fallback to message id if id is not found', async () => {
-//     await locale.set('en')
-//     expect(_('batatinha.quente')).toBe('batatinha.quente')
-//   })
-
-//   it('should fallback to default value if id is not found', async () => {
-//     await locale.set('en')
-//     expect(_('batatinha.quente', { default: 'Hot Potato' })).toBe('Hot Potato')
-//   })
-
-//   it('should fallback to generic locale XX if id not found in XX-YY', async () => {
-//     await locale.set('en-GB')
-//     expect(_('sneakers', { locale: 'en-GB' })).toBe('trainers')
-//   })
-
-//   it('should fallback to generic locale XX if id not found in XX-YY', async () => {
-//     await locale.set('en-GB')
-//     expect(_('switch.lang')).toBe('Switch language')
-//   })
-
-//   it('should accept single object with id prop as the message path', async () => {
-//     await locale.set('en')
-//     expect(_({ id: 'switch.lang' })).toBe('Switch language')
-//   })
-
-//   it('should translate to passed locale', async () => {
-//     await waitLocale('pt-BR')
-//     expect(_('switch.lang', { locale: 'pt' })).toBe('Trocar idioma')
-//   })
-
-//   it('should interpolate message with variables', async () => {
-//     await locale.set('en')
-//     expect(_('greeting.message', { values: { name: 'Chris' } })).toBe(
-//       'Hello Chris, how are you?'
-//     )
-//   })
-// })
-
-// describe('utilities', () => {
-
-//   describe('format utils', () => {
-//     beforeAll(async () => {
-//       await locale.set('en')
-//     })
-
-//     it('should capital a translated message', () => {
-//       expect(_.capital('hi')).toBe('Hi yo')
-//     })
-
-//     it('should title a translated message', () => {
-//       expect(_.title('hi')).toBe('Hi Yo')
-//     })
-
-//     it('should lowercase a translated message', () => {
-//       expect(_.lower('hi')).toBe('hi yo')
-//     })
-
-//     it('should uppercase a translated message', () => {
-//       expect(_.upper('hi')).toBe('HI YO')
-//     })
-
-//     const date = new Date(2019, 3, 24, 23, 45)
-//     it('should format a time value', async () => {
-//       await locale.set('en')
-//       expect(_.time(date)).toBe('11:45 PM')
-//       expect(_.time(date, { format: 'medium' })).toBe('11:45:00 PM')
-//       expect(_.time(date, { format: 'medium', locale: 'pt-BR' })).toBe(
-//         '23:45:00'
-//       )
-//     })
-
-//     it('should format a date value', () => {
-//       expect(_.date(date)).toBe('4/24/19')
-//       expect(_.date(date, { format: 'medium' })).toBe('Apr 24, 2019')
-//     })
-//     // number
-//     it('should format a date value', () => {
-//       expect(_.number(123123123)).toBe('123,123,123')
-//     })
-//   })
-// })
-
-// describe('custom formats', () => {
-//   it('should format messages with custom formats', async () => {
-//     addCustomFormats({
-//       number: {
-//         usd: { style: 'currency', currency: 'USD' },
-//         brl: { style: 'currency', currency: 'BRL' },
-//       },
-//       date: {
-//         customDate: { year: 'numeric', era: 'short' },
-//       },
-//       time: {
-//         customTime: { hour: '2-digit', minute: '2-digit' },
-//       },
-//     })
-
-//     await locale.set('en-US')
-
-//     expect(_.number(123123123, { format: 'usd' })).toContain('$123,123,123.00')
-
-//     expect(_.date(new Date(2019, 0, 1), { format: 'customDate' })).toEqual(
-//       '2019 AD'
-//     )
-
-//     expect(
-//       _.time(new Date(2019, 0, 1, 2, 0, 0), { format: 'customTime' })
-//     ).toEqual('02:00')
-//   })
-// })
diff --git a/test/client/stores/dictionary.test.ts b/test/client/stores/dictionary.test.ts
index 8541932..2af7f9d 100644
--- a/test/client/stores/dictionary.test.ts
+++ b/test/client/stores/dictionary.test.ts
@@ -1,3 +1,5 @@
+import { get } from 'svelte/store'
+
 import {
   getDictionary,
   hasLocaleDictionary,
@@ -8,7 +10,6 @@ import {
   $locales,
   getLocaleDictionary,
 } from '../../../src/client/stores/dictionary'
-import { get } from 'svelte/store'
 
 beforeEach(() => {
   $dictionary.set({})
diff --git a/test/client/stores/locale.test.ts b/test/client/stores/locale.test.ts
index 953ba9f..01583ae 100644
--- a/test/client/stores/locale.test.ts
+++ b/test/client/stores/locale.test.ts
@@ -1,5 +1,6 @@
-import { lookupMessage } from './../../../src/client/includes/lookup'
 import { get } from 'svelte/store'
+import { lookupMessage } from '../../../src/client/includes/lookup'
+
 
 import {
   isFallbackLocaleOf,
diff --git a/test/fixtures/formats.json b/test/fixtures/formats.json
index d0e1edd..d36cac1 100644
--- a/test/fixtures/formats.json
+++ b/test/fixtures/formats.json
@@ -7,6 +7,10 @@
     "customDate": { "year": "numeric", "era": "short" }
   },
   "time": {
-    "customTime": { "hour": "2-digit", "minute": "2-digit" }
+    "customTime": {
+      "hour": "2-digit",
+      "minute": "2-digit",
+      "second": "2-digit"
+    }
   }
 }