1 files changed, 129 insertions, 0 deletions
diff --git a/date-fns/src/addDays/test.ts b/date-fns/src/addDays/test.ts
new file mode 100644
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+++ b/date-fns/src/addDays/test.ts
@@ -0,0 +1,129 @@
+// @flow
+/* eslint-env mocha */
+
+import assert from 'assert'
+import addDays from '.'
+import { getDstTransitions } from '../../test/dst/tzOffsetTransitions'
+
+describe('addDays', function() {
+  it('adds the given number of days', function() {
+    const result = addDays(new Date(2014, 8 /* Sep */, 1), 10)
+    assert.deepStrictEqual(result, new Date(2014, 8 /* Sep */, 11))
+  })
+
+  it('accepts a timestamp', function() {
+    const result = addDays(new Date(2014, 8 /* Sep */, 1).getTime(), 10)
+    assert.deepStrictEqual(result, new Date(2014, 8 /* Sep */, 11))
+  })
+
+  it('converts a fractional number to an integer', function() {
+    const result = addDays(new Date(2014, 8 /* Sep */, 1), 10.5)
+    assert.deepStrictEqual(result, new Date(2014, 8 /* Sep */, 11))
+  })
+
+  it('implicitly converts number arguments', function() {
+    // @ts-expect-error
+    const result = addDays(new Date(2014, 8 /* Sep */, 1), '10')
+    assert.deepStrictEqual(result, new Date(2014, 8 /* Sep */, 11))
+  })
+
+  it('does not mutate the original date', function() {
+    const date = new Date(2014, 8 /* Sep */, 1)
+    addDays(date, 11)
+    assert.deepStrictEqual(date, new Date(2014, 8 /* Sep */, 1))
+  })
+
+  it('returns `Invalid Date` if the given date is invalid', function() {
+    const result = addDays(new Date(NaN), 10)
+    assert(result instanceof Date && isNaN(result.getTime()))
+  })
+
+  it('returns `Invalid Date` if the given amount is NaN', function() {
+    const result = addDays(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 () {
+    // @ts-expect-error
+    assert.throws(addDays.bind(null), TypeError)
+    // @ts-expect-error
+    assert.throws(addDays.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 MINUTE = 1000 * 60
+  // It's usually 1 hour, but for some timezones, e.g. Australia/Lord_Howe, it is 30 minutes
+  const dstOffset = dstTransitions.start && dstTransitions.end ? ((dstTransitions.end.getTimezoneOffset() - dstTransitions.start.getTimezoneOffset()) * MINUTE) : NaN
+
+  dstOnly(
+    `works at DST-start boundary in local timezone: ${tz || '(unknown)'}`,
+    function() {
+      const date = dstTransitions.start
+      const result = addDays(date!, 1)
+      assert.deepStrictEqual(result, new Date(date!.getTime() + 24 * HOUR))
+    }
+  )
+
+  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 = addDays(date, 1)
+      // started before the transition so will only be 23 hours later in local time
+      assert.deepStrictEqual(result, new Date(date.getTime() + 24 * HOUR - dstOffset))
+    }
+  )
+
+  dstOnly(
+    `works at DST-start - 60 mins in local timezone: ${tz || '(unknown)'}`,
+    function() {
+      const date = new Date(dstTransitions.start!.getTime() - 1 * HOUR)
+      const result = addDays(date, 1)
+      // started before the transition so will only be 23 hours later in local time
+      assert.deepStrictEqual(result, new Date(date.getTime() + 24 * HOUR - dstOffset))
+    }
+  )
+
+  dstOnly(
+    `works at DST-end boundary in local timezone: ${tz || '(unknown)'}`,
+    function() {
+      const date = dstTransitions.end
+      const result = addDays(date!, 1)
+      assert.deepStrictEqual(result, new Date(date!.getTime() + 24 * HOUR))
+    }
+  )
+
+  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 = addDays(date, 1)
+      // started before the transition so will be 25 hours later in local
+      // time because one hour repeats after DST ends.
+      assert.deepStrictEqual(result, new Date(date.getTime() + 24 * HOUR + dstOffset))
+    }
+  )
+
+  dstOnly(
+    `works at DST-end - 60 mins in local timezone: ${tz || '(unknown)'}`,
+    function() {
+      const date = new Date(dstTransitions.end!.getTime() - 1 * HOUR)
+      const result = addDays(date, 1)
+      // started before the transition so will be 25 hours later in local
+      // time because one hour repeats after DST ends.
+      assert.deepStrictEqual(result, new Date(date.getTime() + 24 * HOUR + dstOffset))
+    }
+  )
+
+  dstOnly(
+    `doesn't mutate if zero increment is used: ${tz || '(unknown)'}`,
+    function() {
+      const date = new Date(dstTransitions.end!)
+      const result = addDays(date, 0)
+      assert.deepStrictEqual(result, date)
+    }
+  )
+})