5 files changed, 140 insertions, 0 deletions
diff --git a/date-fns/src/startOfDecade/benchmark.js b/date-fns/src/startOfDecade/benchmark.js
new file mode 100644
index 0000000..2f3acb5
--- /dev/null
+++ b/date-fns/src/startOfDecade/benchmark.js
@@ -0,0 +1,15 @@
+// @flow
+/* eslint-env mocha */
+/* global suite, benchmark */
+
+import startOfDecade from '.'
+
+suite('startOfDecade', function () {
+  benchmark('date-fns', function () {
+    return startOfDecade(this.date)
+  })
+}, {
+  setup: function () {
+    this.date = new Date()
+  }
+})
diff --git a/date-fns/src/startOfDecade/index.d.ts b/date-fns/src/startOfDecade/index.d.ts
new file mode 100644
index 0000000..c4309c3
--- /dev/null
+++ b/date-fns/src/startOfDecade/index.d.ts
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+// This file is generated automatically by `scripts/build/typings.js`. Please, don't change it.
+
+import { startOfDecade } from 'date-fns'
+export default startOfDecade
diff --git a/date-fns/src/startOfDecade/index.js.flow b/date-fns/src/startOfDecade/index.js.flow
new file mode 100644
index 0000000..4b7c616
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+++ b/date-fns/src/startOfDecade/index.js.flow
@@ -0,0 +1,52 @@
+// @flow
+// This file is generated automatically by `scripts/build/typings.js`. Please, don't change it.
+
+export type Interval = {
+  start: Date | number,
+  end: Date | number,
+}
+
+export type Locale = {
+  code?: string,
+  formatDistance?: (...args: Array<any>) => any,
+  formatRelative?: (...args: Array<any>) => any,
+  localize?: {
+    ordinalNumber: (...args: Array<any>) => any,
+    era: (...args: Array<any>) => any,
+    quarter: (...args: Array<any>) => any,
+    month: (...args: Array<any>) => any,
+    day: (...args: Array<any>) => any,
+    dayPeriod: (...args: Array<any>) => any,
+  },
+  formatLong?: {
+    date: (...args: Array<any>) => any,
+    time: (...args: Array<any>) => any,
+    dateTime: (...args: Array<any>) => any,
+  },
+  match?: {
+    ordinalNumber: (...args: Array<any>) => any,
+    era: (...args: Array<any>) => any,
+    quarter: (...args: Array<any>) => any,
+    month: (...args: Array<any>) => any,
+    day: (...args: Array<any>) => any,
+    dayPeriod: (...args: Array<any>) => any,
+  },
+  options?: {
+    weekStartsOn?: 0 | 1 | 2 | 3 | 4 | 5 | 6,
+    firstWeekContainsDate?: 1 | 2 | 3 | 4 | 5 | 6 | 7,
+  },
+}
+
+export type Duration = {
+  years?: number,
+  months?: number,
+  weeks?: number,
+  days?: number,
+  hours?: number,
+  minutes?: number,
+  seconds?: number,
+}
+
+export type Day = 0 | 1 | 2 | 3 | 4 | 5 | 6
+
+declare module.exports: (date: Date | number) => Date
diff --git a/date-fns/src/startOfDecade/index.ts b/date-fns/src/startOfDecade/index.ts
new file mode 100644
index 0000000..fc6c0ac
--- /dev/null
+++ b/date-fns/src/startOfDecade/index.ts
@@ -0,0 +1,34 @@
+import toDate from '../toDate/index'
+import requiredArgs from '../_lib/requiredArgs/index'
+
+/**
+ * @name startOfDecade
+ * @category Decade Helpers
+ * @summary Return the start of a decade for the given date.
+ *
+ * @description
+ * Return the start of a decade for the given date.
+ *
+ * ### v2.0.0 breaking changes:
+ *
+ * - [Changes that are common for the whole library](https://github.com/date-fns/date-fns/blob/master/docs/upgradeGuide.md#Common-Changes).
+ *
+ * @param {Date|Number} date - the original date
+ * @returns {Date} the start of a decade
+ * @throws {TypeError} 1 argument required
+ *
+ * @example
+ * // The start of a decade for 21 October 2015 00:00:00:
+ * const result = startOfDecade(new Date(2015, 9, 21, 00, 00, 00))
+ * //=> Jan 01 2010 00:00:00
+ */
+export default function startOfDecade(dirtyDate: Date | number): Date {
+  requiredArgs(1, arguments)
+
+  const date = toDate(dirtyDate)
+  const year = date.getFullYear()
+  const decade = Math.floor(year / 10) * 10
+  date.setFullYear(decade, 0, 1)
+  date.setHours(0, 0, 0, 0)
+  return date
+}
diff --git a/date-fns/src/startOfDecade/test.ts b/date-fns/src/startOfDecade/test.ts
new file mode 100644
index 0000000..b9f7dee
--- /dev/null
+++ b/date-fns/src/startOfDecade/test.ts
@@ -0,0 +1,35 @@
+// @flow
+/* eslint-env mocha */
+
+import assert from 'power-assert'
+import startOfDecade from '.'
+
+describe('startOfDecade', function() {
+  it('returns the date with the time set to 00:00:00 and the date set to the first day of a year', function() {
+    const date = new Date(1953, 3 /* Apr */, 13)
+    const result = startOfDecade(date)
+    assert.deepEqual(result, new Date(1950, 0 /* Jan */, 1))
+  })
+
+  it('accepts a timestamp', function() {
+    const date = new Date(1984, 9 /* Oct */, 14).getTime()
+    const result = startOfDecade(date)
+    assert.deepEqual(result, new Date(1980, 0 /* Jan */, 1))
+  })
+
+  it('does not mutate the original date', function() {
+    const date = new Date(1978, 10 /* Nov */, 14)
+    startOfDecade(date)
+    assert.deepEqual(date, new Date(1978, 10 /* Nov */, 14))
+  })
+
+  it('returns `Invalid Date` if the given date is invalid', function() {
+    const result = startOfDecade(new Date(NaN))
+    //@ts-expect-error
+    assert(result instanceof Date && isNaN(result))
+  })
+
+  it('throws TypeError exception if passed less than 1 argument', function() {
+    assert.throws(startOfDecade.bind(null), TypeError)
+  })
+})