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diff --git a/date-fns/src/startOfWeek/test.ts b/date-fns/src/startOfWeek/test.ts
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+++ b/date-fns/src/startOfWeek/test.ts
@@ -0,0 +1,113 @@
+// @flow
+/* eslint-env mocha */
+
+import assert from 'assert'
+import startOfWeek from '.'
+
+describe('startOfWeek', 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 () {
+    const date = new Date(2014, 8 /* Sep */, 2, 11, 55, 0)
+    const result = startOfWeek(date)
+    assert.deepEqual(result, new Date(2014, 7 /* Aug */, 31))
+  })
+
+  it('allows to specify which day is the first day of the week', function () {
+    const date = new Date(2014, 8 /* Sep */, 2, 11, 55, 0)
+    const result = startOfWeek(date, { weekStartsOn: 1 })
+    assert.deepEqual(result, new Date(2014, 8 /* Sep */, 1))
+  })
+
+  it('allows to specify which day is the first day of the week in locale', function () {
+    const date = new Date(2014, 8 /* Sep */, 2, 11, 55, 0)
+    const result = startOfWeek(date, {
+      // @ts-expect-error
+      locale: {
+        options: { weekStartsOn: 1 },
+      },
+    })
+    assert.deepEqual(result, new Date(2014, 8 /* Sep */, 1))
+  })
+
+  it('`options.weekStartsOn` overwrites the first day of the week specified in locale', function () {
+    const date = new Date(2014, 8 /* Sep */, 2, 11, 55, 0)
+    const result = startOfWeek(date, {
+      weekStartsOn: 1,
+      // @ts-expect-error
+      locale: {
+        options: { weekStartsOn: 0 },
+      },
+    })
+    assert.deepEqual(result, new Date(2014, 8 /* Sep */, 1))
+  })
+
+  it('implicitly converts options', function () {
+    const date = new Date(2014, 8 /* Sep */, 2, 11, 55, 0)
+    // @ts-expect-error
+    const result = startOfWeek(date, { weekStartsOn: '1' })
+    assert.deepEqual(result, new Date(2014, 8 /* Sep */, 1))
+  })
+
+  it('accepts a timestamp', function () {
+    const date = new Date(2014, 8 /* Sep */, 2, 11, 55, 0).getTime()
+    const result = startOfWeek(date)
+    assert.deepEqual(result, new Date(2014, 7 /* Aug */, 31))
+  })
+
+  it('does not mutate the original date', function () {
+    const date = new Date(2014, 8 /* Sep */, 2, 11, 55, 0)
+    startOfWeek(date)
+    assert.deepEqual(date, new Date(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 () {
+        const date = new Date(2014, 9 /* Oct */, 6)
+        const result = startOfWeek(date, { weekStartsOn: 3 })
+        assert.deepEqual(result, new Date(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 () {
+        const date = new Date(2014, 9 /* Oct */, 8)
+        const result = startOfWeek(date, { weekStartsOn: 3 })
+        assert.deepEqual(result, new Date(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 () {
+        const date = new Date(2014, 9 /* Oct */, 10)
+        const result = startOfWeek(date, { weekStartsOn: 3 })
+        assert.deepEqual(result, new Date(2014, 9 /* Oct */, 8))
+      })
+    })
+
+    it('handles the week at the start of a year', function () {
+      const date = new Date(2014, 0 /* Jan */, 1)
+      const result = startOfWeek(date)
+      assert.deepEqual(result, new Date(2013, 11 /* Dec */, 29))
+    })
+  })
+
+  it('returns `Invalid Date` if the given date is invalid', function () {
+    const result = startOfWeek(new Date(NaN))
+    assert(result instanceof Date && isNaN(Number(result)))
+  })
+
+  it('throws `RangeError` if `options.weekStartsOn` is not convertable to 0, 1, ..., 6 or undefined', function () {
+    const block = () =>
+      startOfWeek(
+        new Date(2014, 8 /* Sep */, 2, 11, 55, 0),
+        // @ts-expect-error
+        { weekStartsOn: NaN }
+      )
+    assert.throws(block, RangeError)
+  })
+
+  it('throws TypeError exception if passed less than 1 argument', function () {
+    // @ts-expect-error
+    assert.throws(startOfWeek.bind(null), TypeError)
+  })
+})