diff --git a/src/getWeekYear/index.js b/src/getWeekYear/index.ts
similarity index 76%
rename from src/getWeekYear/index.js
rename to src/getWeekYear/index.ts
index 08a9437d96..ce31d5d27e 100644
--- a/src/getWeekYear/index.js
+++ b/src/getWeekYear/index.ts
@@ -2,6 +2,11 @@ import startOfWeek from '../startOfWeek/index'
 import toDate from '../toDate/index'
 import toInteger from '../_lib/toInteger/index'
 import requiredArgs from '../_lib/requiredArgs/index'
+import {
+  WeekStartOptions,
+  LocaleOptions,
+  FirstWeekContainsDateOptions,
+} from '../types'
 
 /**
  * @name getWeekYear
@@ -33,34 +38,36 @@ import requiredArgs from '../_lib/requiredArgs/index'
  *
  * @example
  * // Which week numbering year is 26 December 2004 with the default settings?
- * var result = getWeekYear(new Date(2004, 11, 26))
+ * const result = getWeekYear(new Date(2004, 11, 26))
  * //=> 2005
  *
  * @example
  * // Which week numbering year is 26 December 2004 if week starts on Saturday?
- * var result = getWeekYear(new Date(2004, 11, 26), { weekStartsOn: 6 })
+ * const result = getWeekYear(new Date(2004, 11, 26), { weekStartsOn: 6 })
  * //=> 2004
  *
  * @example
  * // Which week numbering year is 26 December 2004 if the first week contains 4 January?
- * var result = getWeekYear(new Date(2004, 11, 26), { firstWeekContainsDate: 4 })
+ * const result = getWeekYear(new Date(2004, 11, 26), { firstWeekContainsDate: 4 })
  * //=> 2004
  */
-export default function getWeekYear(dirtyDate, dirtyOptions) {
+export default function getWeekYear(
+  dirtyDate: Date | number,
+  options: LocaleOptions & WeekStartOptions & FirstWeekContainsDateOptions = {}
+): number {
   requiredArgs(1, arguments)
 
-  var date = toDate(dirtyDate)
-  var year = date.getFullYear()
+  const date = toDate(dirtyDate)
+  const year = date.getFullYear()
 
-  var options = dirtyOptions || {}
-  var locale = options.locale
-  var localeFirstWeekContainsDate =
+  const locale = options.locale
+  const localeFirstWeekContainsDate =
     locale && locale.options && locale.options.firstWeekContainsDate
-  var defaultFirstWeekContainsDate =
+  const defaultFirstWeekContainsDate =
     localeFirstWeekContainsDate == null
       ? 1
       : toInteger(localeFirstWeekContainsDate)
-  var firstWeekContainsDate =
+  const firstWeekContainsDate =
     options.firstWeekContainsDate == null
       ? defaultFirstWeekContainsDate
       : toInteger(options.firstWeekContainsDate)
@@ -72,15 +79,15 @@ export default function getWeekYear(dirtyDate, dirtyOptions) {
     )
   }
 
-  var firstWeekOfNextYear = new Date(0)
+  const firstWeekOfNextYear = new Date(0)
   firstWeekOfNextYear.setFullYear(year + 1, 0, firstWeekContainsDate)
   firstWeekOfNextYear.setHours(0, 0, 0, 0)
-  var startOfNextYear = startOfWeek(firstWeekOfNextYear, dirtyOptions)
+  const startOfNextYear = startOfWeek(firstWeekOfNextYear, options)
 
-  var firstWeekOfThisYear = new Date(0)
+  const firstWeekOfThisYear = new Date(0)
   firstWeekOfThisYear.setFullYear(year, 0, firstWeekContainsDate)
   firstWeekOfThisYear.setHours(0, 0, 0, 0)
-  var startOfThisYear = startOfWeek(firstWeekOfThisYear, dirtyOptions)
+  const startOfThisYear = startOfWeek(firstWeekOfThisYear, options)
 
   if (date.getTime() >= startOfNextYear.getTime()) {
     return year + 1
diff --git a/src/getWeekYear/test.js b/src/getWeekYear/test.js
deleted file mode 100644
index 5272931c9f..0000000000
--- a/src/getWeekYear/test.js
+++ /dev/null
@@ -1,74 +0,0 @@
-// @flow
-/* eslint-env mocha */
-
-import assert from 'power-assert'
-import getWeekYear from '.'
-
-describe('getWeekYear', function() {
-  it('returns the local week-numbering year of the given date', function() {
-    var result = getWeekYear(new Date(2004, 11 /* Dec */, 26))
-    assert(result === 2005)
-  })
-
-  it('accepts a timestamp', function() {
-    var result = getWeekYear(new Date(2000, 11 /* Dec */, 30).getTime())
-    assert(result === 2000)
-  })
-
-  it('handles dates before 100 AD', function() {
-    var initialDate = new Date(0)
-    initialDate.setFullYear(7, 11 /* Dec */, 31)
-    initialDate.setHours(0, 0, 0, 0)
-    var result = getWeekYear(initialDate)
-    assert(result === 8)
-  })
-
-  it('returns NaN if the given date is invalid', function() {
-    var result = getWeekYear(new Date(NaN))
-    assert(isNaN(result))
-  })
-
-  it('allows to specify `weekStartsOn` and `firstWeekContainsDate` in locale', function() {
-    var date = new Date(2004, 11 /* Dec */, 26)
-    var result = getWeekYear(date, {
-      // $ExpectedMistake
-      locale: {
-        options: { weekStartsOn: 1, firstWeekContainsDate: 4 }
-      }
-    })
-    assert(result === 2004)
-  })
-
-  it('`options.weekStartsOn` overwrites the first day of the week specified in locale', function() {
-    var date = new Date(2004, 11 /* Dec */, 26)
-    var result = getWeekYear(date, {
-      weekStartsOn: 1,
-      firstWeekContainsDate: 4,
-      // $ExpectedMistake
-      locale: {
-        options: { weekStartsOn: 0, firstWeekContainsDate: 1 }
-      }
-    })
-    assert(result === 2004)
-  })
-
-  it('throws `RangeError` if `options.weekStartsOn` is not convertable to 0, 1, ..., 6 or undefined', function() {
-    // $ExpectedMistake
-    var block = getWeekYear.bind(null, new Date(2007, 11 /* Dec */, 31), {
-      weekStartsOn: NaN
-    })
-    assert.throws(block, RangeError)
-  })
-
-  it('throws `RangeError` if `options.firstWeekContainsDate` is not convertable to 1, 2, ..., 7 or undefined', function() {
-    // $ExpectedMistake
-    var block = getWeekYear.bind(null, new Date(2007, 11 /* Dec */, 31), {
-      firstWeekContainsDate: NaN
-    })
-    assert.throws(block, RangeError)
-  })
-
-  it('throws TypeError exception if passed less than 1 argument', function() {
-    assert.throws(getWeekYear.bind(null), TypeError)
-  })
-})
diff --git a/src/getWeekYear/test.ts b/src/getWeekYear/test.ts
new file mode 100644
index 0000000000..0f8bf54902
--- /dev/null
+++ b/src/getWeekYear/test.ts
@@ -0,0 +1,75 @@
+// @flow
+/* eslint-env mocha */
+
+import assert from 'assert'
+import getWeekYear from '.'
+
+describe('getWeekYear', () => {
+  it('returns the local week-numbering year of the given date', () => {
+    const result = getWeekYear(new Date(2004, 11 /* Dec */, 26))
+    assert(result === 2005)
+  })
+
+  it('accepts a timestamp', () => {
+    const result = getWeekYear(new Date(2000, 11 /* Dec */, 30).getTime())
+    assert(result === 2000)
+  })
+
+  it('handles dates before 100 AD', () => {
+    const initialDate = new Date(0)
+    initialDate.setFullYear(7, 11 /* Dec */, 31)
+    initialDate.setHours(0, 0, 0, 0)
+    const result = getWeekYear(initialDate)
+    assert(result === 8)
+  })
+
+  it('returns NaN if the given date is invalid', () => {
+    const result = getWeekYear(new Date(NaN))
+    assert(isNaN(result))
+  })
+
+  it('allows to specify `weekStartsOn` and `firstWeekContainsDate` in locale', () => {
+    const date = new Date(2004, 11 /* Dec */, 26)
+    const result = getWeekYear(date, {
+      // @ts-expect-error
+      locale: {
+        options: { weekStartsOn: 1, firstWeekContainsDate: 4 },
+      },
+    })
+    assert(result === 2004)
+  })
+
+  it('`options.weekStartsOn` overwrites the first day of the week specified in locale', () => {
+    const date = new Date(2004, 11 /* Dec */, 26)
+    const result = getWeekYear(date, {
+      weekStartsOn: 1,
+      firstWeekContainsDate: 4,
+      // @ts-expect-error
+      locale: {
+        options: { weekStartsOn: 0, firstWeekContainsDate: 1 },
+      },
+    })
+    assert(result === 2004)
+  })
+
+  it('throws `RangeError` if `options.weekStartsOn` is not convertable to 0, 1, ..., 6 or undefined', () => {
+    // @ts-expect-error
+    const block = getWeekYear.bind(null, new Date(2007, 11 /* Dec */, 31), {
+      weekStartsOn: NaN,
+    })
+    assert.throws(block, RangeError)
+  })
+
+  it('throws `RangeError` if `options.firstWeekContainsDate` is not convertable to 1, 2, ..., 7 or undefined', () => {
+    // @ts-expect-error
+    const block = getWeekYear.bind(null, new Date(2007, 11 /* Dec */, 31), {
+      firstWeekContainsDate: NaN,
+    })
+    assert.throws(block, RangeError)
+  })
+
+  it('throws TypeError exception if passed less than 1 argument', () => {
+    // @ts-expect-error
+    assert.throws(getWeekYear.bind(null), TypeError)
+  })
+})
diff --git a/src/types.ts b/src/types.ts
index 751e10b60e..1ccc4658a2 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -27,8 +27,14 @@ export interface LocaleOptions {
   locale?: Locale
 }
 
+export interface FirstWeekContainsDateOptions {
+  firstWeekContainsDate?: FirstWeekContainsDate
+}
+
 export type Day = 0 | 1 | 2 | 3 | 4 | 5 | 6
 
+export type FirstWeekContainsDate = 1 | 4
+
 export interface DateValues {
   year?: number
   month?: number