1 files changed, 173 insertions, 0 deletions
diff --git a/date-fns/src/addMonths/test.ts b/date-fns/src/addMonths/test.ts
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+++ b/date-fns/src/addMonths/test.ts
@@ -0,0 +1,173 @@
+// @flow
+/* eslint-env mocha */
+
+import assert from 'assert'
+import addMonths from '.'
+import { getDstTransitions } from '../../test/dst/tzOffsetTransitions'
+
+describe('addMonths', function() {
+  it('adds the given number of months', function() {
+    const result = addMonths(new Date(2014, 8 /* Sep */, 1), 5)
+    assert.deepStrictEqual(result, new Date(2015, 1 /* Feb */, 1))
+  })
+
+  it('accepts a timestamp', function() {
+    const result = addMonths(new Date(2014, 8 /* Sep */, 1).getTime(), 12)
+    assert.deepStrictEqual(result, new Date(2015, 8 /* Sep */, 1))
+  })
+
+  it('converts a fractional number to an integer', function() {
+    const result = addMonths(new Date(2014, 8 /* Sep */, 1), 5.75)
+    assert.deepStrictEqual(result, new Date(2015, 1 /* Feb */, 1))
+  })
+
+  it('implicitly converts number arguments', function() {
+    // @ts-expect-error
+    const result = addMonths(new Date(2014, 8 /* Sep */, 1), '5')
+    assert.deepStrictEqual(result, new Date(2015, 1 /* Feb */, 1))
+  })
+
+  it('does not mutate the original date', function() {
+    const date = new Date(2014, 8 /* Sep */, 1)
+    addMonths(date, 12)
+    assert.deepStrictEqual(date, new Date(2014, 8 /* Sep */, 1))
+  })
+
+  it('works well if the desired month has fewer days and the provided date is in the last day of a month', function() {
+    const date = new Date(2014, 11 /* Dec */, 31)
+    const result = addMonths(date, 2)
+    assert.deepStrictEqual(result, new Date(2015, 1 /* Feb */, 28))
+  })
+
+  it('handles dates before 100 AD', function() {
+    const initialDate = new Date(0)
+    initialDate.setFullYear(0, 0 /* Jan */, 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 = addMonths(initialDate, 1)
+    assert.deepStrictEqual(result, expectedResult)
+  })
+
+  it('returns `Invalid Date` if the given date is invalid', function() {
+    const result = addMonths(new Date(NaN), 5)
+    // @ts-expect-error
+    assert(result instanceof Date && isNaN(result))
+  })
+
+  it('returns `Invalid Date` if the given amount is NaN', function() {
+    const result = addMonths(new Date(2014, 8 /* Sep */, 1), NaN)
+    // @ts-expect-error
+    assert(result instanceof Date && isNaN(result))
+  })
+
+  it('throws TypeError exception if passed less than 2 arguments', function() {
+    // @ts-expect-error
+    assert.throws(addMonths.bind(null), TypeError)
+    // @ts-expect-error
+    assert.throws(addMonths.bind(null, 1), TypeError)
+  })
+
+  const dstTransitions = getDstTransitions(2017)
+  const dstOnly = dstTransitions.start && dstTransitions.end ? it : it.skip
+  const tz = Intl.DateTimeFormat().resolvedOptions().timeZone || process.env.tz
+  const HOUR = 1000 * 60 * 60
+  const override = (
+    base: Date,
+    year = base.getFullYear(),
+    month = base.getMonth(),
+    day = base.getDate(),
+    hour = base.getHours(),
+    minute = base.getMinutes()
+  ) => new Date(year, month, day, hour, minute)
+
+  dstOnly(
+    `works at DST-start boundary in local timezone: ${tz || '(unknown)'}`,
+    function() {
+      const date = dstTransitions.start
+      const result = addMonths(date!, 2)
+      assert.deepStrictEqual(
+        result,
+        override(date!, date!.getFullYear(), date!.getMonth() + 2)
+      )
+    }
+  )
+
+  dstOnly(
+    `works at DST-start - 30 mins in local timezone: ${tz || '(unknown)'}`,
+    function() {
+      const date = new Date(dstTransitions.start!.getTime() - 0.5 * HOUR)
+      const result = addMonths(date, 2)
+      const expected = override(date, date.getFullYear(), date.getMonth() + 2)
+      assert.deepStrictEqual(result, expected)
+    }
+  )
+
+  dstOnly(
+    `works at DST-start - 60 mins in local timezone: ${tz || '(unknown)'}`,
+    function() {
+      const date = new Date(dstTransitions.start!.getTime() - 1 * HOUR)
+      const result = addMonths(date, 2)
+      const expected = override(date, date.getFullYear(), date.getMonth() + 2)
+      assert.deepStrictEqual(result, expected)
+    }
+  )
+
+  dstOnly(
+    `works at DST-end boundary in local timezone: ${tz || '(unknown)'}`,
+    function() {
+      const date = dstTransitions.end
+      const result = addMonths(date!, 2)
+      assert.deepStrictEqual(
+        result,
+        override(
+          date!,
+          date!.getFullYear() + (date!.getMonth() >= 10 ? 1 : 0),
+          (date!.getMonth() + 2) % 12 // protect against wrap for Nov.
+        )
+      )
+    }
+  )
+
+  dstOnly(
+    `works at DST-end - 30 mins in local timezone: ${tz || '(unknown)'}`,
+    function() {
+      const date = new Date(dstTransitions.end!.getTime() - 0.5 * HOUR)
+      const result = addMonths(date, 2)
+      assert.deepStrictEqual(
+        result,
+        override(
+          date,
+          date.getFullYear() + (date.getMonth() >= 10 ? 1 : 0),
+          (date.getMonth() + 2) % 12 // protect against wrap for Nov.
+        )
+      )
+    }
+  )
+
+  dstOnly(
+    `works at DST-end - 60 mins in local timezone: ${tz || '(unknown)'}`,
+    function() {
+      const date = new Date(dstTransitions.end!.getTime() - 1 * HOUR)
+      const result = addMonths(date, 2)
+      assert.deepStrictEqual(
+        result,
+        override(
+          date,
+          date.getFullYear() + (date.getMonth() >= 10 ? 1 : 0),
+          (date.getMonth() + 2) % 12 // protect against wrap for Nov.
+        )
+      )
+    }
+  )
+
+  dstOnly(
+    `doesn't mutate if zero increment is used: ${tz || '(unknown)'}`,
+    function() {
+      const date = new Date(dstTransitions.end!)
+      const result = addMonths(date, 0)
+      assert.deepStrictEqual(result, date)
+    }
+  )
+})