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+// @flow
+/* eslint-env mocha */
+
+import assert from 'power-assert'
+import startOfUTCWeek from '.'
+
+describe('startOfUTCWeek', function () {
+  it('returns the date with the time set to 00:00:00 and the date set to the first day of a week', function () {
+    var date = new Date(Date.UTC(2014, 8 /* Sep */, 2, 11, 55, 0))
+    var result = startOfUTCWeek(date)
+    assert.deepEqual(result, new Date(Date.UTC(2014, 7 /* Aug */, 31)))
+  })
+
+  it('allows to specify which day is the first day of the week', function () {
+    var date = new Date(Date.UTC(2014, 8 /* Sep */, 2, 11, 55, 0))
+    var result = startOfUTCWeek(date, { weekStartsOn: 1 })
+    assert.deepEqual(result, new Date(Date.UTC(2014, 8 /* Sep */, 1)))
+  })
+
+  it('allows to specify which day is the first day of the week in locale', function () {
+    var date = new Date(Date.UTC(2014, 8 /* Sep */, 2, 11, 55, 0))
+    var result = startOfUTCWeek(date, {
+      locale: {
+        options: { weekStartsOn: 1 },
+      },
+    })
+    assert.deepEqual(result, new Date(Date.UTC(2014, 8 /* Sep */, 1)))
+  })
+
+  it('`options.weekStartsOn` overwrites the first day of the week specified in locale', function () {
+    var date = new Date(Date.UTC(2014, 8 /* Sep */, 2, 11, 55, 0))
+    var result = startOfUTCWeek(date, {
+      weekStartsOn: 1,
+      locale: {
+        options: { weekStartsOn: 0 },
+      },
+    })
+    assert.deepEqual(result, new Date(Date.UTC(2014, 8 /* Sep */, 1)))
+  })
+
+  it('implicitly converts options', function () {
+    var date = new Date(Date.UTC(2014, 8 /* Sep */, 2, 11, 55, 0))
+    var result = startOfUTCWeek(date, { weekStartsOn: '1' })
+    assert.deepEqual(result, new Date(Date.UTC(2014, 8 /* Sep */, 1)))
+  })
+
+  it('accepts a timestamp', function () {
+    var date = new Date(Date.UTC(2014, 8 /* Sep */, 2, 11, 55, 0)).getTime()
+    var result = startOfUTCWeek(date)
+    assert.deepEqual(result, new Date(Date.UTC(2014, 7 /* Aug */, 31)))
+  })
+
+  it('does not mutate the original date', function () {
+    var date = new Date(Date.UTC(2014, 8 /* Sep */, 2, 11, 55, 0))
+    startOfUTCWeek(date)
+    assert.deepEqual(date, new Date(Date.UTC(2014, 8 /* Sep */, 2, 11, 55, 0)))
+  })
+
+  describe('edge cases', function () {
+    describe('when the given day is before the start of a week', function () {
+      it('it returns the start of a week', function () {
+        var date = new Date(Date.UTC(2014, 9 /* Oct */, 6))
+        var result = startOfUTCWeek(date, { weekStartsOn: 3 })
+        assert.deepEqual(result, new Date(Date.UTC(2014, 9 /* Oct */, 1)))
+      })
+    })
+
+    describe('when the given day is the start of a week', function () {
+      it('it returns the start of a week', function () {
+        var date = new Date(Date.UTC(2014, 9 /* Oct */, 8))
+        var result = startOfUTCWeek(date, { weekStartsOn: 3 })
+        assert.deepEqual(result, new Date(Date.UTC(2014, 9 /* Oct */, 8)))
+      })
+    })
+
+    describe('when the given day is after the start of a week', function () {
+      it('it returns the start of a week', function () {
+        var date = new Date(Date.UTC(2014, 9 /* Oct */, 10))
+        var result = startOfUTCWeek(date, { weekStartsOn: 3 })
+        assert.deepEqual(result, new Date(Date.UTC(2014, 9 /* Oct */, 8)))
+      })
+    })
+
+    it('handles the week at the start of a year', function () {
+      var date = new Date(Date.UTC(2014, 0 /* Jan */, 1))
+      var result = startOfUTCWeek(date)
+      assert.deepEqual(result, new Date(Date.UTC(2013, 11 /* Dec */, 29)))
+    })
+  })
+
+  it('returns `Invalid Date` if the given date is invalid', function () {
+    var result = startOfUTCWeek(new Date(NaN))
+    assert(result instanceof Date && isNaN(result))
+  })
+
+  it('throws `RangeError` if `options.weekStartsOn` is not convertable to 0, 1, ..., 6 or undefined', function () {
+    var block = startOfUTCWeek.bind(
+      null,
+      new Date(Date.UTC(2014, 8 /* Sep */, 2, 11, 55, 0)),
+      { weekStartsOn: NaN }
+    )
+    assert.throws(block, RangeError)
+  })
+
+  it('throws TypeError exception if passed less than 1 argument', function () {
+    assert.throws(startOfUTCWeek.bind(null), TypeError)
+  })
+})