diff --git a/src/formatDistanceStrict/index.js b/src/formatDistanceStrict/index.ts
similarity index 81%
rename from src/formatDistanceStrict/index.js
rename to src/formatDistanceStrict/index.ts
index 0c0d1d08b5..d792e7337b 100644
--- a/src/formatDistanceStrict/index.js
+++ b/src/formatDistanceStrict/index.ts
@@ -4,11 +4,12 @@ import toDate from '../toDate/index'
 import cloneObject from '../_lib/cloneObject/index'
 import defaultLocale from '../locale/en-US/index'
 import requiredArgs from '../_lib/requiredArgs/index'
+import { LocaleOptions, Unit } from '../types';
 
-var MILLISECONDS_IN_MINUTE = 1000 * 60
-var MINUTES_IN_DAY = 60 * 24
-var MINUTES_IN_MONTH = MINUTES_IN_DAY * 30
-var MINUTES_IN_YEAR = MINUTES_IN_DAY * 365
+const MILLISECONDS_IN_MINUTE = 1000 * 60
+const MINUTES_IN_DAY = 60 * 24
+const MINUTES_IN_MONTH = MINUTES_IN_DAY * 30
+const MINUTES_IN_YEAR = MINUTES_IN_DAY * 365
 
 /**
  * @name formatDistanceStrict
@@ -115,13 +116,13 @@ var MINUTES_IN_YEAR = MINUTES_IN_DAY * 365
  *
  * @example
  * // What is the distance between 2 July 2014 and 1 January 2015?
- * var result = formatDistanceStrict(new Date(2014, 6, 2), new Date(2015, 0, 2))
+ * const result = formatDistanceStrict(new Date(2014, 6, 2), new Date(2015, 0, 2))
  * //=> '6 months'
  *
  * @example
  * // What is the distance between 1 January 2015 00:00:15
  * // and 1 January 2015 00:00:00?
- * var result = formatDistanceStrict(
+ * const result = formatDistanceStrict(
  *   new Date(2015, 0, 1, 0, 0, 15),
  *   new Date(2015, 0, 1, 0, 0, 0)
  * )
@@ -130,7 +131,7 @@ var MINUTES_IN_YEAR = MINUTES_IN_DAY * 365
  * @example
  * // What is the distance from 1 January 2016
  * // to 1 January 2015, with a suffix?
- * var result = formatDistanceStrict(new Date(2015, 0, 1), new Date(2016, 0, 1), {
+ * const result = formatDistanceStrict(new Date(2015, 0, 1), new Date(2016, 0, 1), {
  *   addSuffix: true
  * })
  * //=> '1 year ago'
@@ -138,7 +139,7 @@ var MINUTES_IN_YEAR = MINUTES_IN_DAY * 365
  * @example
  * // What is the distance from 1 January 2016
  * // to 1 January 2015, in minutes?
- * var result = formatDistanceStrict(new Date(2016, 0, 1), new Date(2015, 0, 1), {
+ * const result = formatDistanceStrict(new Date(2016, 0, 1), new Date(2015, 0, 1), {
  *   unit: 'minute'
  * })
  * //=> '525600 minutes'
@@ -146,7 +147,7 @@ var MINUTES_IN_YEAR = MINUTES_IN_DAY * 365
  * @example
  * // What is the distance from 1 January 2015
  * // to 28 January 2015, in months, rounded up?
- * var result = formatDistanceStrict(new Date(2015, 0, 28), new Date(2015, 0, 1), {
+ * const result = formatDistanceStrict(new Date(2015, 0, 28), new Date(2015, 0, 1), {
  *   unit: 'month',
  *   roundingMethod: 'ceil'
  * })
@@ -155,37 +156,41 @@ var MINUTES_IN_YEAR = MINUTES_IN_DAY * 365
  * @example
  * // What is the distance between 1 August 2016 and 1 January 2015 in Esperanto?
  * import { eoLocale } from 'date-fns/locale/eo'
- * var result = formatDistanceStrict(new Date(2016, 7, 1), new Date(2015, 0, 1), {
+ * const result = formatDistanceStrict(new Date(2016, 7, 1), new Date(2015, 0, 1), {
  *   locale: eoLocale
  * })
  * //=> '1 jaro'
  */
+
 export default function formatDistanceStrict(
-  dirtyDate,
-  dirtyBaseDate,
-  dirtyOptions
-) {
+  dirtyDate: Date | number,
+  dirtyBaseDate: Date | number,
+  options: LocaleOptions & {
+    addSuffix?: boolean,
+    unit?: Unit,
+    roundingMethod?: 'floor' | 'ceil' | 'round',
+  } = {}
+): string {
   requiredArgs(2, arguments)
 
-  var options = dirtyOptions || {}
-  var locale = options.locale || defaultLocale
+  const locale = options.locale || defaultLocale
 
   if (!locale.formatDistance) {
     throw new RangeError('locale must contain localize.formatDistance property')
   }
 
-  var comparison = compareAsc(dirtyDate, dirtyBaseDate)
+  const comparison = compareAsc(dirtyDate, dirtyBaseDate)
 
   if (isNaN(comparison)) {
     throw new RangeError('Invalid time value')
   }
 
-  var localizeOptions = cloneObject(options)
+  const localizeOptions = cloneObject(options)
   localizeOptions.addSuffix = Boolean(options.addSuffix)
   localizeOptions.comparison = comparison
 
-  var dateLeft
-  var dateRight
+  let dateLeft
+  let dateRight
   if (comparison > 0) {
     dateLeft = toDate(dirtyBaseDate)
     dateRight = toDate(dirtyDate)
@@ -194,9 +199,9 @@ export default function formatDistanceStrict(
     dateRight = toDate(dirtyBaseDate)
   }
 
-  var roundingMethod =
+  const roundingMethod =
     options.roundingMethod == null ? 'round' : String(options.roundingMethod)
-  var roundingMethodFn
+  let roundingMethodFn
 
   if (roundingMethod === 'floor') {
     roundingMethodFn = Math.floor
@@ -208,19 +213,19 @@ export default function formatDistanceStrict(
     throw new RangeError("roundingMethod must be 'floor', 'ceil' or 'round'")
   }
 
-  var milliseconds = dateRight.getTime() - dateLeft.getTime()
-  var minutes = milliseconds / MILLISECONDS_IN_MINUTE
+  const milliseconds = dateRight.getTime() - dateLeft.getTime()
+  const minutes = milliseconds / MILLISECONDS_IN_MINUTE
 
-  var timezoneOffset =
+  const timezoneOffset =
     getTimezoneOffsetInMilliseconds(dateRight) -
     getTimezoneOffsetInMilliseconds(dateLeft)
 
   // Use DST-normalized difference in minutes for years, months and days;
   // use regular difference in minutes for hours, minutes and seconds.
-  var dstNormalizedMinutes =
+  const dstNormalizedMinutes =
     (milliseconds - timezoneOffset) / MILLISECONDS_IN_MINUTE
 
-  var unit
+  let unit
   if (options.unit == null) {
     if (minutes < 1) {
       unit = 'second'
@@ -241,34 +246,34 @@ export default function formatDistanceStrict(
 
   // 0 up to 60 seconds
   if (unit === 'second') {
-    var seconds = roundingMethodFn(milliseconds / 1000)
+    const seconds = roundingMethodFn(milliseconds / 1000)
     return locale.formatDistance('xSeconds', seconds, localizeOptions)
 
     // 1 up to 60 mins
   } else if (unit === 'minute') {
-    var roundedMinutes = roundingMethodFn(minutes)
+    const roundedMinutes = roundingMethodFn(minutes)
     return locale.formatDistance('xMinutes', roundedMinutes, localizeOptions)
 
     // 1 up to 24 hours
   } else if (unit === 'hour') {
-    var hours = roundingMethodFn(minutes / 60)
+    const hours = roundingMethodFn(minutes / 60)
     return locale.formatDistance('xHours', hours, localizeOptions)
 
     // 1 up to 30 days
   } else if (unit === 'day') {
-    var days = roundingMethodFn(dstNormalizedMinutes / MINUTES_IN_DAY)
+    const days = roundingMethodFn(dstNormalizedMinutes / MINUTES_IN_DAY)
     return locale.formatDistance('xDays', days, localizeOptions)
 
     // 1 up to 12 months
   } else if (unit === 'month') {
-    var months = roundingMethodFn(dstNormalizedMinutes / MINUTES_IN_MONTH)
+    const months = roundingMethodFn(dstNormalizedMinutes / MINUTES_IN_MONTH)
     return months === 12 && options.unit !== 'month'
       ? locale.formatDistance('xYears', 1, localizeOptions)
       : locale.formatDistance('xMonths', months, localizeOptions)
 
     // 1 year up to max Date
   } else if (unit === 'year') {
-    var years = roundingMethodFn(dstNormalizedMinutes / MINUTES_IN_YEAR)
+    const years = roundingMethodFn(dstNormalizedMinutes / MINUTES_IN_YEAR)
     return locale.formatDistance('xYears', years, localizeOptions)
   }
 
diff --git a/src/formatDistanceStrict/test.js b/src/formatDistanceStrict/test.ts
similarity index 82%
rename from src/formatDistanceStrict/test.js
rename to src/formatDistanceStrict/test.ts
index 52d94c8475..52bb2bda5b 100644
--- a/src/formatDistanceStrict/test.js
+++ b/src/formatDistanceStrict/test.ts
@@ -1,14 +1,13 @@
-// @flow
 /* eslint-env mocha */
 
-import assert from 'power-assert'
+import assert from 'assert'
 import formatDistanceStrict from '.'
 
 describe('formatDistanceStrict', function () {
   describe('seconds', function () {
     describe('when no unit is set', function () {
       it('0 seconds', function () {
-        var result = formatDistanceStrict(
+        const result = formatDistanceStrict(
           new Date(1986, 3, 4, 10, 32, 5),
           new Date(1986, 3, 4, 10, 32, 5)
         )
@@ -16,7 +15,7 @@ describe('formatDistanceStrict', function () {
       })
 
       it('5 seconds', function () {
-        var result = formatDistanceStrict(
+        const result = formatDistanceStrict(
           new Date(1986, 3, 4, 10, 32, 0),
           new Date(1986, 3, 4, 10, 32, 5)
         )
@@ -27,7 +26,7 @@ describe('formatDistanceStrict', function () {
 
   describe('minutes', function () {
     it('1 minute', function () {
-      var result = formatDistanceStrict(
+      const result = formatDistanceStrict(
         new Date(1986, 3, 4, 10, 32, 0),
         new Date(1986, 3, 4, 10, 33, 0)
       )
@@ -35,7 +34,7 @@ describe('formatDistanceStrict', function () {
     })
 
     it('n minutes', function () {
-      var result = formatDistanceStrict(
+      const result = formatDistanceStrict(
         new Date(1986, 3, 4, 10, 32, 0),
         new Date(1986, 3, 4, 10, 35, 0)
       )
@@ -45,7 +44,7 @@ describe('formatDistanceStrict', function () {
 
   describe('hours', function () {
     it('1 hour', function () {
-      var result = formatDistanceStrict(
+      const result = formatDistanceStrict(
         new Date(1986, 3, 4, 10, 32, 0),
         new Date(1986, 3, 4, 11, 32, 0)
       )
@@ -53,7 +52,7 @@ describe('formatDistanceStrict', function () {
     })
 
     it('n hours', function () {
-      var result = formatDistanceStrict(
+      const result = formatDistanceStrict(
         new Date(1986, 3, 4, 10, 32, 0),
         new Date(1986, 3, 4, 13, 32, 0)
       )
@@ -63,7 +62,7 @@ describe('formatDistanceStrict', function () {
 
   describe('days', function () {
     it('1 day', function () {
-      var result = formatDistanceStrict(
+      const result = formatDistanceStrict(
         new Date(1986, 3, 4, 10, 32, 0),
         new Date(1986, 3, 5, 10, 32, 0)
       )
@@ -71,7 +70,7 @@ describe('formatDistanceStrict', function () {
     })
 
     it('n days', function () {
-      var result = formatDistanceStrict(
+      const result = formatDistanceStrict(
         new Date(1986, 3, 4, 10, 32, 0),
         new Date(1986, 3, 7, 10, 32, 0)
       )
@@ -81,7 +80,7 @@ describe('formatDistanceStrict', function () {
 
   describe('months', function () {
     it('1 month', function () {
-      var result = formatDistanceStrict(
+      const result = formatDistanceStrict(
         new Date(1986, 3, 4, 10, 32, 0),
         new Date(1986, 4, 4, 10, 32, 0)
       )
@@ -89,7 +88,7 @@ describe('formatDistanceStrict', function () {
     })
 
     it('n months', function () {
-      var result = formatDistanceStrict(
+      const result = formatDistanceStrict(
         new Date(1986, 3, 4, 10, 32, 0),
         new Date(1986, 6, 4, 10, 32, 0)
       )
@@ -115,7 +114,7 @@ describe('formatDistanceStrict', function () {
     })
 
     it('1 year', function () {
-      var result = formatDistanceStrict(
+      const result = formatDistanceStrict(
         new Date(1986, 3, 4, 10, 32, 0),
         new Date(1987, 3, 4, 10, 32, 0)
       )
@@ -123,7 +122,7 @@ describe('formatDistanceStrict', function () {
     })
 
     it('n years', function () {
-      var result = formatDistanceStrict(
+      const result = formatDistanceStrict(
         new Date(1986, 3, 4, 10, 32, 0),
         new Date(1991, 3, 4, 10, 32, 0)
       )
@@ -134,7 +133,7 @@ describe('formatDistanceStrict', function () {
   describe('when the unit option is supplied', function () {
     describe('second', function () {
       it('0 seconds', function () {
-        var result = formatDistanceStrict(
+        const result = formatDistanceStrict(
           new Date(1986, 3, 4, 10, 32, 0),
           new Date(1986, 3, 4, 10, 32, 0),
           { unit: 'second' }
@@ -143,7 +142,7 @@ describe('formatDistanceStrict', function () {
       })
 
       it('5 seconds', function () {
-        var result = formatDistanceStrict(
+        const result = formatDistanceStrict(
           new Date(1986, 3, 4, 10, 32, 0),
           new Date(1986, 3, 4, 10, 32, 5),
           { unit: 'second' }
@@ -152,7 +151,7 @@ describe('formatDistanceStrict', function () {
       })
 
       it('120 seconds', function () {
-        var result = formatDistanceStrict(
+        const result = formatDistanceStrict(
           new Date(1986, 3, 4, 10, 32, 0),
           new Date(1986, 3, 4, 10, 34, 0),
           { unit: 'second' }
@@ -163,7 +162,7 @@ describe('formatDistanceStrict', function () {
 
     describe('minute', function () {
       it('0 minutes', function () {
-        var result = formatDistanceStrict(
+        const result = formatDistanceStrict(
           new Date(1986, 3, 4, 10, 32, 0),
           new Date(1986, 3, 4, 10, 32, 0),
           { unit: 'minute' }
@@ -172,7 +171,7 @@ describe('formatDistanceStrict', function () {
       })
 
       it('5 minutes', function () {
-        var result = formatDistanceStrict(
+        const result = formatDistanceStrict(
           new Date(1986, 3, 4, 10, 32, 0),
           new Date(1986, 3, 4, 10, 37, 0),
           { unit: 'minute' }
@@ -181,7 +180,7 @@ describe('formatDistanceStrict', function () {
       })
 
       it('120 minutes', function () {
-        var result = formatDistanceStrict(
+        const result = formatDistanceStrict(
           new Date(1986, 3, 4, 10, 32, 0),
           new Date(1986, 3, 4, 12, 32, 0),
           { unit: 'minute' }
@@ -192,7 +191,7 @@ describe('formatDistanceStrict', function () {
 
     describe('hour', function () {
       it('0 hours', function () {
-        var result = formatDistanceStrict(
+        const result = formatDistanceStrict(
           new Date(1986, 3, 4, 10, 32, 0),
           new Date(1986, 3, 4, 10, 32, 0),
           { unit: 'hour' }
@@ -201,7 +200,7 @@ describe('formatDistanceStrict', function () {
       })
 
       it('5 hours', function () {
-        var result = formatDistanceStrict(
+        const result = formatDistanceStrict(
           new Date(1986, 3, 4, 10, 32, 0),
           new Date(1986, 3, 4, 15, 32, 0),
           { unit: 'hour' }
@@ -210,7 +209,7 @@ describe('formatDistanceStrict', function () {
       })
 
       it('48 hours', function () {
-        var result = formatDistanceStrict(
+        const result = formatDistanceStrict(
           new Date(1986, 3, 4, 10, 32, 0),
           new Date(1986, 3, 6, 10, 32, 0),
           { unit: 'hour' }
@@ -221,7 +220,7 @@ describe('formatDistanceStrict', function () {
 
     describe('day', function () {
       it('0 days', function () {
-        var result = formatDistanceStrict(
+        const result = formatDistanceStrict(
           new Date(1986, 3, 4, 10, 32, 0),
           new Date(1986, 3, 4, 10, 32, 0),
           { unit: 'day' }
@@ -230,7 +229,7 @@ describe('formatDistanceStrict', function () {
       })
 
       it('5 days', function () {
-        var result = formatDistanceStrict(
+        const result = formatDistanceStrict(
           new Date(1986, 3, 4, 10, 32, 0),
           new Date(1986, 3, 9, 10, 32, 0),
           { unit: 'day' }
@@ -239,7 +238,7 @@ describe('formatDistanceStrict', function () {
       })
 
       it('60 days', function () {
-        var result = formatDistanceStrict(
+        const result = formatDistanceStrict(
           new Date(1986, 3, 4, 10, 32, 0),
           new Date(1986, 5, 3, 10, 32, 0),
           { unit: 'day' }
@@ -249,7 +248,7 @@ describe('formatDistanceStrict', function () {
     })
     describe('month', function () {
       it('0 months', function () {
-        var result = formatDistanceStrict(
+        const result = formatDistanceStrict(
           new Date(1986, 3, 4, 10, 32, 0),
           new Date(1986, 3, 4, 10, 32, 0),
           { unit: 'month' }
@@ -258,7 +257,7 @@ describe('formatDistanceStrict', function () {
       })
 
       it('5 months', function () {
-        var result = formatDistanceStrict(
+        const result = formatDistanceStrict(
           new Date(1986, 3, 4, 10, 32, 0),
           new Date(1986, 7, 4, 10, 32, 0),
           { unit: 'month' }
@@ -267,7 +266,7 @@ describe('formatDistanceStrict', function () {
       })
 
       it('12 months - see issue 2388', function () {
-        var result = formatDistanceStrict(
+        const result = formatDistanceStrict(
           new Date(1986, 7, 4, 10, 32, 0),
           new Date(1985, 7, 4, 10, 32, 0),
           { unit: 'month' }
@@ -276,7 +275,7 @@ describe('formatDistanceStrict', function () {
       })
 
       it('24 months', function () {
-        var result = formatDistanceStrict(
+        const result = formatDistanceStrict(
           new Date(1986, 3, 4, 10, 32, 0),
           new Date(1988, 3, 4, 10, 32, 0),
           { unit: 'month' }
@@ -287,7 +286,7 @@ describe('formatDistanceStrict', function () {
 
     describe('year', function () {
       it('0 years', function () {
-        var result = formatDistanceStrict(
+        const result = formatDistanceStrict(
           new Date(1986, 3, 4, 10, 32, 0),
           new Date(1986, 3, 4, 10, 32, 0),
           { unit: 'year' }
@@ -296,7 +295,7 @@ describe('formatDistanceStrict', function () {
       })
 
       it('5 years', function () {
-        var result = formatDistanceStrict(
+        const result = formatDistanceStrict(
           new Date(1986, 3, 4, 10, 32, 0),
           new Date(1991, 3, 4, 15, 32, 0),
           { unit: 'year' }
@@ -307,7 +306,7 @@ describe('formatDistanceStrict', function () {
   })
 
   it('accepts timestamps', function () {
-    var result = formatDistanceStrict(
+    const result = formatDistanceStrict(
       new Date(1986, 3, 4, 10, 32, 0).getTime(),
       new Date(1986, 3, 4, 11, 32, 0).getTime()
     )
@@ -316,7 +315,7 @@ describe('formatDistanceStrict', function () {
 
   describe('when the addSuffix option is true', function () {
     it('adds a past suffix', function () {
-      var result = formatDistanceStrict(
+      const result = formatDistanceStrict(
         new Date(1986, 3, 4, 10, 32, 0),
         new Date(1986, 3, 4, 10, 32, 25),
         { addSuffix: true }
@@ -325,7 +324,7 @@ describe('formatDistanceStrict', function () {
     })
 
     it('adds a future suffix', function () {
-      var result = formatDistanceStrict(
+      const result = formatDistanceStrict(
         new Date(1986, 3, 4, 11, 32, 0),
         new Date(1986, 3, 4, 10, 32, 0),
         { addSuffix: true }
@@ -336,7 +335,7 @@ describe('formatDistanceStrict', function () {
 
   describe('when the roundingMethod option is supplied', function () {
     it('default is "round"', function () {
-      var result = formatDistanceStrict(
+      const result = formatDistanceStrict(
         new Date(1986, 3, 4, 10, 32, 0),
         new Date(1986, 3, 4, 10, 33, 59)
       )
@@ -344,7 +343,7 @@ describe('formatDistanceStrict', function () {
     })
 
     it('"floor"', function () {
-      var result = formatDistanceStrict(
+      const result = formatDistanceStrict(
         new Date(1986, 3, 4, 10, 32, 0),
         new Date(1986, 3, 4, 10, 33, 59),
         { roundingMethod: 'floor' }
@@ -353,7 +352,7 @@ describe('formatDistanceStrict', function () {
     })
 
     it('"ceil"', function () {
-      var result = formatDistanceStrict(
+      const result = formatDistanceStrict(
         new Date(1986, 3, 4, 10, 32, 0),
         new Date(1986, 3, 4, 10, 33, 1),
         { roundingMethod: 'ceil' }
@@ -362,7 +361,7 @@ describe('formatDistanceStrict', function () {
     })
 
     it('"round" (down)', function () {
-      var result = formatDistanceStrict(
+      const result = formatDistanceStrict(
         new Date(1986, 3, 4, 10, 32, 0),
         new Date(1986, 3, 4, 10, 33, 29),
         { roundingMethod: 'round' }
@@ -371,7 +370,7 @@ describe('formatDistanceStrict', function () {
     })
 
     it('"round" (up)', function () {
-      var result = formatDistanceStrict(
+      const result = formatDistanceStrict(
         new Date(1986, 3, 4, 10, 32, 0),
         new Date(1986, 3, 4, 10, 33, 30),
         { roundingMethod: 'round' }
@@ -383,22 +382,22 @@ describe('formatDistanceStrict', function () {
   describe('implicit conversion of options', function () {
     it('`options.unit`', function () {
       // eslint-disable-next-line no-new-wrappers
-      var unit = new String('year')
+      const unit = new String('year')
 
-      var result = formatDistanceStrict(
+      const result = formatDistanceStrict(
         new Date(1986, 3, 4, 10, 32, 0),
         new Date(1986, 3, 4, 10, 32, 0),
-        // $ExpectedMistake
+        // @ts-expect-error
         { unit: unit }
       )
       assert(result === '0 years')
     })
 
     it('`options.addSuffix`', function () {
-      var result = formatDistanceStrict(
+      const result = formatDistanceStrict(
         new Date(1986, 3, 4, 10, 32, 0),
         new Date(1986, 3, 4, 10, 32, 25),
-        // $ExpectedMistake
+        // @ts-expect-error
         { addSuffix: 1 }
       )
       assert(result === '25 seconds ago')
@@ -406,12 +405,12 @@ describe('formatDistanceStrict', function () {
 
     it('`options.ceil`', function () {
       // eslint-disable-next-line no-new-wrappers
-      var roundingMethod = new String('ceil')
+      const roundingMethod = new String('ceil')
 
-      var result = formatDistanceStrict(
+      const result = formatDistanceStrict(
         new Date(1986, 3, 4, 10, 32, 0),
         new Date(1986, 3, 4, 10, 33, 1),
-        // $ExpectedMistake
+        // @ts-expect-error
         { roundingMethod: roundingMethod }
       )
       assert(result === '2 minutes')
@@ -420,7 +419,7 @@ describe('formatDistanceStrict', function () {
 
   describe('custom locale', function () {
     it('can be passed to the function', function () {
-      function localizeDistance(token, count, options) {
+      function localizeDistance(token: string, count: number, options: { addSuffix: boolean; comparison: number }) {
         assert(token === 'xSeconds')
         assert(count === 25)
         assert(options.addSuffix === true)
@@ -428,14 +427,14 @@ describe('formatDistanceStrict', function () {
         return 'It works!'
       }
 
-      var customLocale = {
+      const customLocale = {
         formatDistance: localizeDistance,
       }
 
-      var result = formatDistanceStrict(
+      const result = formatDistanceStrict(
         new Date(1986, 3, 4, 10, 32, 0),
         new Date(1986, 3, 4, 10, 32, 25),
-        // $ExpectedMistake
+        // @ts-expect-error
         { addSuffix: true, locale: customLocale }
       )
 
@@ -444,11 +443,11 @@ describe('formatDistanceStrict', function () {
 
     describe('does not contain `formatDistance` property', function () {
       it('throws `RangeError`', function () {
-        var customLocale = {}
-        var block = formatDistanceStrict.bind(
+        const customLocale = {}
+        // @ts-expect-error
+        const block = formatDistanceStrict.bind(
           null,
           new Date(1986, 3, 4, 10, 32, 0),
-          // $ExpectedMistake
           new Date(1986, 3, 4, 10, 37, 0),
           { unit: 'minute', locale: customLocale }
         )
@@ -459,7 +458,7 @@ describe('formatDistanceStrict', function () {
 
   describe('edge cases', function () {
     it('detects unit correctly for short months', function () {
-      var result = formatDistanceStrict(
+      const result = formatDistanceStrict(
         new Date(2018, 1 /* Feb */, 1),
         new Date(2018, 2 /* Mar */, 1)
       )
@@ -497,29 +496,29 @@ describe('formatDistanceStrict', function () {
   })
 
   it("throws `RangeError` if `options.roundingMethod` is not 'floor', 'ceil', 'round' or undefined", function () {
-    var block = formatDistanceStrict.bind(
-      null,
+    const block = () => formatDistanceStrict(
       new Date(1986, 3, 4, 10, 32, 0),
       new Date(1986, 3, 4, 10, 33, 29),
-      // $ExpectedMistake
+      // @ts-expect-error
       { roundingMethod: 'foobar' }
     )
     assert.throws(block, RangeError)
   })
 
   it("throws `RangeError` if `options.unit` is not 's', 'm', 'h', 'd', 'M', 'Y' or undefined", function () {
-    var block = formatDistanceStrict.bind(
-      null,
+    const block = () => formatDistanceStrict(
       new Date(1986, 3, 4, 10, 32, 0),
       new Date(1986, 3, 4, 10, 33, 29),
-      // $ExpectedMistake
+      // @ts-expect-error
       { unit: 'foobar' }
     )
     assert.throws(block, RangeError)
   })
 
   it('throws TypeError exception if passed less than 2 arguments', function () {
+    // @ts-expect-error
     assert.throws(formatDistanceStrict.bind(null), TypeError)
+    // @ts-expect-error
     assert.throws(formatDistanceStrict.bind(null, 1), TypeError)
   })
 })
diff --git a/src/types.ts b/src/types.ts
index 1ccc4658a2..39b9c2e952 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -44,3 +44,10 @@ export interface DateValues {
   seconds?: number
   milliseconds?: number
 }
+
+export type Unit = 'second'
+  | 'minute'
+  | 'hour'
+  | 'day'
+  | 'month'
+  | 'year';