5 files changed, 186 insertions, 0 deletions
diff --git a/date-fns/src/setMonth/benchmark.js b/date-fns/src/setMonth/benchmark.js
new file mode 100644
index 0000000..0113f1f
--- /dev/null
+++ b/date-fns/src/setMonth/benchmark.js
@@ -0,0 +1,21 @@
+// @flow
+/* eslint-env mocha */
+/* global suite, benchmark */
+
+import setMonth from '.'
+import moment from 'moment'
+
+suite('setMonth', function () {
+  benchmark('date-fns', function () {
+    return setMonth(this.date, 6)
+  })
+
+  benchmark('Moment.js', function () {
+    return this.moment.month(6)
+  })
+}, {
+  setup: function () {
+    this.date = new Date()
+    this.moment = moment()
+  }
+})
diff --git a/date-fns/src/setMonth/index.d.ts b/date-fns/src/setMonth/index.d.ts
new file mode 100644
index 0000000..720d61c
--- /dev/null
+++ b/date-fns/src/setMonth/index.d.ts
@@ -0,0 +1,4 @@
+// This file is generated automatically by `scripts/build/typings.js`. Please, don't change it.
+
+import { setMonth } from 'date-fns'
+export default setMonth
diff --git a/date-fns/src/setMonth/index.js.flow b/date-fns/src/setMonth/index.js.flow
new file mode 100644
index 0000000..29bd4de
--- /dev/null
+++ b/date-fns/src/setMonth/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, month: number) => Date
diff --git a/date-fns/src/setMonth/index.ts b/date-fns/src/setMonth/index.ts
new file mode 100644
index 0000000..c625378
--- /dev/null
+++ b/date-fns/src/setMonth/index.ts
@@ -0,0 +1,44 @@
+import toInteger from '../_lib/toInteger/index'
+import toDate from '../toDate/index'
+import getDaysInMonth from '../getDaysInMonth/index'
+import requiredArgs from '../_lib/requiredArgs/index'
+
+/**
+ * @name setMonth
+ * @category Month Helpers
+ * @summary Set the month to the given date.
+ *
+ * @description
+ * Set the month to 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 date to be changed
+ * @param {Number} month - the month of the new date
+ * @returns {Date} the new date with the month set
+ * @throws {TypeError} 2 arguments required
+ *
+ * @example
+ * // Set February to 1 September 2014:
+ * const result = setMonth(new Date(2014, 8, 1), 1)
+ * //=> Sat Feb 01 2014 00:00:00
+ */
+export default function setMonth(dirtyDate: Date | number, dirtyMonth: number): Date {
+  requiredArgs(2, arguments)
+
+  const date = toDate(dirtyDate)
+  const month = toInteger(dirtyMonth)
+  const year = date.getFullYear()
+  const day = date.getDate()
+
+  const dateWithDesiredMonth = new Date(0)
+  dateWithDesiredMonth.setFullYear(year, month, 15)
+  dateWithDesiredMonth.setHours(0, 0, 0, 0)
+  const daysInMonth = getDaysInMonth(dateWithDesiredMonth)
+  // Set the last day of the new month
+  // if the original date was the last day of the longer month
+  date.setMonth(month, Math.min(day, daysInMonth))
+  return date
+}
diff --git a/date-fns/src/setMonth/test.ts b/date-fns/src/setMonth/test.ts
new file mode 100644
index 0000000..75d61db
--- /dev/null
+++ b/date-fns/src/setMonth/test.ts
@@ -0,0 +1,65 @@
+// @flow
+/* eslint-env mocha */
+
+import assert from 'power-assert'
+import setMonth from '.'
+
+describe('setMonth', function() {
+  it('sets the month', function() {
+    const result = setMonth(new Date(2014, 8 /* Sep */, 1), 1)
+    assert.deepEqual(result, new Date(2014, 1 /* Feb */, 1))
+  })
+
+  it('sets the last day of the month if the original date was the last day of a longer month', function() {
+    const result = setMonth(new Date(2014, 11 /* Dec */, 31), 1)
+    assert.deepEqual(result, new Date(2014, 1 /* Feb */, 28))
+  })
+
+  it('accepts a timestamp', function() {
+    const result = setMonth(new Date(2014, 8 /* Sep */, 1).getTime(), 11)
+    assert.deepEqual(result, new Date(2014, 11 /* Dec */, 1))
+  })
+
+  it('converts a fractional number to an integer', function() {
+    const result = setMonth(new Date(2014, 8 /* Sep */, 1), 1.5)
+    assert.deepEqual(result, new Date(2014, 1 /* Feb */, 1))
+  })
+
+  it('implicitly converts number arguments', function() {
+    // @ts-expect-error
+    const result = setMonth(new Date(2014, 8 /* Sep */, 1), '1')
+    assert.deepEqual(result, new Date(2014, 1 /* Feb */, 1))
+  })
+
+  it('does not mutate the original date', function() {
+    const date = new Date(2014, 8 /* Sep */, 1)
+    setMonth(date, 5)
+    assert.deepEqual(date, new Date(2014, 8 /* Sep */, 1))
+  })
+
+  it('handles dates before 100 AD', function() {
+    const initialDate = new Date(0)
+    initialDate.setFullYear(0, 11 /* Dec */, 31)
+    initialDate.setHours(0, 0, 0, 0)
+    const expectedResult = new Date(0)
+    expectedResult.setFullYear(0, 1 /* Feb */, 29)
+    expectedResult.setHours(0, 0, 0, 0)
+    const result = setMonth(initialDate, 1)
+    assert.deepEqual(result, expectedResult)
+  })
+
+  it('returns `Invalid Date` if the given date is invalid', function() {
+    const result = setMonth(new Date(NaN), 1)
+    assert(result instanceof Date && isNaN(result.getTime()))
+  })
+
+  it('returns `Invalid Date` if the given amount is NaN', function() {
+    const result = setMonth(new Date(2014, 8 /* Sep */, 1), NaN)
+    assert(result instanceof Date && isNaN(result.getTime()))
+  })
+
+  it('throws TypeError exception if passed less than 2 arguments', function() {
+    assert.throws(setMonth.bind(null), TypeError)
+    assert.throws(setMonth.bind(null, 1), TypeError)
+  })
+})