diff --git a/src/formatRelative/index.js b/src/formatRelative/index.ts
similarity index 80%
rename from src/formatRelative/index.js
rename to src/formatRelative/index.ts
index f3f14f49ba..254c94514a 100644
--- a/src/formatRelative/index.js
+++ b/src/formatRelative/index.ts
@@ -5,6 +5,7 @@ import subMilliseconds from '../subMilliseconds/index'
 import toDate from '../toDate/index'
 import getTimezoneOffsetInMilliseconds from '../_lib/getTimezoneOffsetInMilliseconds/index'
 import requiredArgs from '../_lib/requiredArgs/index'
+import { LocaleOptions, WeekStartOptions } from '../types'
 
 /**
  * @name formatRelative
@@ -41,14 +42,16 @@ import requiredArgs from '../_lib/requiredArgs/index'
  * @throws {RangeError} `options.locale` must contain `formatLong` property
  * @throws {RangeError} `options.locale` must contain `formatRelative` property
  */
-export default function formatRelative(dirtyDate, dirtyBaseDate, dirtyOptions) {
+export default function formatRelative(dirtyDate: Date | number, dirtyBaseDate: Date | number, dirtyOptions?: LocaleOptions & WeekStartOptions): string {
   requiredArgs(2, arguments)
 
-  var date = toDate(dirtyDate)
-  var baseDate = toDate(dirtyBaseDate)
+  const date = toDate(dirtyDate)
+  const baseDate = toDate(dirtyBaseDate)
 
-  var options = dirtyOptions || {}
-  var locale = options.locale || defaultLocale
+  const {
+    locale = defaultLocale,
+    weekStartsOn = 0,
+  } = dirtyOptions || {}
 
   if (!locale.localize) {
     throw new RangeError('locale must contain localize property')
@@ -62,13 +65,13 @@ export default function formatRelative(dirtyDate, dirtyBaseDate, dirtyOptions) {
     throw new RangeError('locale must contain formatRelative property')
   }
 
-  var diff = differenceInCalendarDays(date, baseDate)
+  const diff = differenceInCalendarDays(date, baseDate)
 
   if (isNaN(diff)) {
     throw new RangeError('Invalid time value')
   }
 
-  var token
+  let token
   if (diff < -6) {
     token = 'other'
   } else if (diff < -1) {
@@ -85,11 +88,11 @@ export default function formatRelative(dirtyDate, dirtyBaseDate, dirtyOptions) {
     token = 'other'
   }
 
-  var utcDate = subMilliseconds(date, getTimezoneOffsetInMilliseconds(date))
-  var utcBaseDate = subMilliseconds(
+  const utcDate = subMilliseconds(date, getTimezoneOffsetInMilliseconds(date))
+  const utcBaseDate = subMilliseconds(
     baseDate,
     getTimezoneOffsetInMilliseconds(baseDate)
   )
-  var formatStr = locale.formatRelative(token, utcDate, utcBaseDate, options)
-  return format(date, formatStr, options)
+  const formatStr = locale.formatRelative(token, utcDate, utcBaseDate, { locale, weekStartsOn })
+  return format(date, formatStr, { locale, weekStartsOn })
 }
diff --git a/src/formatRelative/test.js b/src/formatRelative/test.ts
similarity index 55%
rename from src/formatRelative/test.js
rename to src/formatRelative/test.ts
index 92ec82bf13..608b67dfa3 100644
--- a/src/formatRelative/test.js
+++ b/src/formatRelative/test.ts
@@ -1,73 +1,72 @@
-// @flow
 /* eslint-env mocha */
 
-import assert from 'power-assert'
+import assert from 'assert'
 import formatRelative from '.'
 
-describe('formatRelative', function() {
-  var baseDate = new Date(1986, 3 /* Apr */, 4, 10, 32, 0, 900)
+describe('formatRelative', function () {
+  const baseDate = new Date(1986, 3 /* Apr */, 4, 10, 32, 0, 900)
 
-  it('accepts a timestamp', function() {
-    var date = new Date(2014, 3 /* Apr */, 4)
+  it('accepts a timestamp', function () {
+    const date = new Date(2014, 3 /* Apr */, 4)
     assert(formatRelative(date.getTime(), baseDate.getTime()) === '04/04/2014')
   })
 
-  it('before the last week', function() {
-    var result = formatRelative(
+  it('before the last week', function () {
+    const result = formatRelative(
       new Date(1986, 2 /* Mar */, 28, 16, 50),
       baseDate
     )
     assert(result === '03/28/1986')
   })
 
-  it('last week', function() {
-    var result = formatRelative(new Date(1986, 3 /* Apr */, 1), baseDate)
+  it('last week', function () {
+    const result = formatRelative(new Date(1986, 3 /* Apr */, 1), baseDate)
     assert(result === 'last Tuesday at 12:00 AM')
   })
 
-  it('yesterday', function() {
-    var result = formatRelative(
+  it('yesterday', function () {
+    const result = formatRelative(
       new Date(1986, 3 /* Apr */, 3, 22, 22),
       baseDate
     )
     assert(result === 'yesterday at 10:22 PM')
   })
 
-  it('today', function() {
-    var result = formatRelative(
+  it('today', function () {
+    const result = formatRelative(
       new Date(1986, 3 /* Apr */, 4, 16, 50),
       baseDate
     )
     assert(result === 'today at 4:50 PM')
   })
 
-  it('tomorrow', function() {
-    var result = formatRelative(new Date(1986, 3 /* Apr */, 5, 7, 30), baseDate)
+  it('tomorrow', function () {
+    const result = formatRelative(new Date(1986, 3 /* Apr */, 5, 7, 30), baseDate)
     assert(result === 'tomorrow at 7:30 AM')
   })
 
-  it('next week', function() {
-    var result = formatRelative(new Date(1986, 3 /* Apr */, 6, 12, 0), baseDate)
+  it('next week', function () {
+    const result = formatRelative(new Date(1986, 3 /* Apr */, 6, 12, 0), baseDate)
     assert(result === 'Sunday at 12:00 PM')
   })
 
-  it('after the next week', function() {
-    var result = formatRelative(
+  it('after the next week', function () {
+    const result = formatRelative(
       new Date(1986, 3 /* Apr */, 11, 16, 50),
       baseDate
     )
     assert(result === '04/11/1986')
   })
 
-  describe('edge cases', function() {
-    it("throws RangeError if the date isn't valid", function() {
+  describe('edge cases', function () {
+    it("throws RangeError if the date isn't valid", function () {
       assert.throws(
         formatRelative.bind(null, new Date(NaN), baseDate),
         RangeError
       )
     })
 
-    it("throws RangeError if the base date isn't valid", function() {
+    it("throws RangeError if the base date isn't valid", function () {
       assert.throws(
         formatRelative.bind(
           null,
@@ -78,92 +77,92 @@ describe('formatRelative', function() {
       )
     })
 
-    it("throws RangeError if both dates aren't valid", function() {
+    it("throws RangeError if both dates aren't valid", function () {
       assert.throws(
         formatRelative.bind(null, new Date(NaN), new Date(NaN)),
         RangeError
       )
     })
 
-    it('handles dates before 100 AD', function() {
-      var date = new Date(0)
+    it('handles dates before 100 AD', function () {
+      const date = new Date(0)
       date.setFullYear(7, 11 /* Dec */, 31)
       date.setHours(0, 0, 0, 0)
       assert(formatRelative(date, baseDate) === '12/31/0007')
     })
   })
 
-  describe('custom locale', function() {
-    it('allows to pass a custom locale', function() {
-      var customLocale = {
+  describe('custom locale', function () {
+    it('allows to pass a custom locale', function () {
+      const customLocale = {
         localize: {
-          month: function() {
+          month: function () {
             return 'works'
           }
         },
         formatLong: {
-          date: function() {
+          date: function () {
             return "'It' MMMM"
           }
         },
-        formatRelative: function() {
+        formatRelative: function () {
           return "P 'perfectly!'"
         }
       }
-      var result = formatRelative(
+      const result = formatRelative(
         new Date(1986, 2 /* Mar */, 28, 16, 50),
         baseDate,
-        // $ExpectedMistake
+        // @ts-expect-error
         { locale: customLocale }
       )
       assert(result === 'It works perfectly!')
     })
 
-    it("throws `RangeError` if `options.locale` doesn't have `localize` property", function() {
-      var customLocale = {
+    it("throws `RangeError` if `options.locale` doesn't have `localize` property", function () {
+      const customLocale = {
         formatLong: {},
-        formatRelative: function() {
+        formatRelative: function () {
           return ''
         }
       }
-      // $ExpectedMistake
-      var block = formatRelative.bind(null, new Date(2017, 0, 1), baseDate, {
+      // @ts-expect-error
+      const block = formatRelative.bind(null, new Date(2017, 0, 1), baseDate, {
         locale: customLocale
       })
       assert.throws(block, RangeError)
     })
 
-    it("throws `RangeError` if `options.locale` doesn't have `formatLong` property", function() {
-      var customLocale = {
-        // $ExpectedMistake
+    it("throws `RangeError` if `options.locale` doesn't have `formatLong` property", function () {
+      const customLocale = {
         localize: {},
-        formatRelative: function() {
+        formatRelative: function () {
           return ''
         }
       }
-      // $ExpectedMistake
-      var block = formatRelative.bind(null, new Date(2017, 0, 1), baseDate, {
+      // @ts-expect-error
+      const block = formatRelative.bind(null, new Date(2017, 0, 1), baseDate, {
         locale: customLocale
       })
       assert.throws(block, RangeError)
     })
 
-    it("throws `RangeError` if `options.locale` doesn't have `formatRelative` property", function() {
-      var customLocale = {
-        // $ExpectedMistake
+    it("throws `RangeError` if `options.locale` doesn't have `formatRelative` property", function () {
+      const customLocale = {
         localize: {},
         formatLong: {}
       }
-      // $ExpectedMistake
-      var block = formatRelative.bind(null, new Date(2017, 0, 1), baseDate, {
+      // @ts-expect-error
+      const block = formatRelative.bind(null, new Date(2017, 0, 1), baseDate, {
         locale: customLocale
       })
       assert.throws(block, RangeError)
     })
   })
 
-  it('throws TypeError exception if passed less than 2 arguments', function() {
+  it('throws TypeError exception if passed less than 2 arguments', function () {
+    // @ts-expect-error
     assert.throws(formatRelative.bind(null), TypeError)
+    // @ts-expect-error
     assert.throws(formatRelative.bind(null, 1), TypeError)
   })
 })