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diff --git a/date-fns/src/startOfWeekYear/test.js b/date-fns/src/startOfWeekYear/test.js
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+// @flow
+/* eslint-env mocha */
+
+import assert from 'power-assert'
+import startOfWeekYear from '.'
+
+describe('startOfWeekYear', function() {
+  it('returns the date with the time set to 00:00:00 and the date set to the first day of a week year', function() {
+    var result = startOfWeekYear(new Date(2005, 6 /* Jul */, 2))
+    assert.deepEqual(result, new Date(2004, 11 /* Dec */, 26, 0, 0, 0, 0))
+  })
+
+  it('accepts a timestamp', function() {
+    var result = startOfWeekYear(new Date(2005, 0 /* Jan */, 1, 6, 0).getTime())
+    assert.deepEqual(result, new Date(2004, 11 /* Dec */, 26, 0, 0, 0, 0))
+  })
+
+  it('does not mutate the original date', function() {
+    var date = new Date(2014, 6 /* Jul */, 2)
+    startOfWeekYear(date)
+    assert.deepEqual(date, new Date(2014, 6 /* Jul */, 2))
+  })
+
+  it('handles dates before 100 AD', function() {
+    var initialDate = new Date(0)
+    initialDate.setFullYear(9, 0 /* Jan */, 1)
+    initialDate.setHours(0, 0, 0, 0)
+    var expectedResult = new Date(0)
+    expectedResult.setFullYear(8, 11 /* Dec */, 28)
+    expectedResult.setHours(0, 0, 0, 0)
+    var result = startOfWeekYear(initialDate)
+    assert.deepEqual(result, expectedResult)
+  })
+
+  it('returns `Invalid Date` if the given date is invalid', function() {
+    var result = startOfWeekYear(new Date(NaN))
+    assert(result instanceof Date && isNaN(result))
+  })
+
+  it('allows to specify `weekStartsOn` and `firstWeekContainsDate` in locale', function() {
+    var date = new Date(2005, 6 /* Jul */, 2)
+    var result = startOfWeekYear(date, {
+      // $ExpectedMistake
+      locale: {
+        options: { weekStartsOn: 1, firstWeekContainsDate: 4 }
+      }
+    })
+    assert.deepEqual(result, new Date(2005, 0 /* Jan */, 3, 0, 0, 0, 0))
+  })
+
+  it('`options.weekStartsOn` overwrites the first day of the week specified in locale', function() {
+    var date = new Date(2005, 6 /* Jul */, 2)
+    var result = startOfWeekYear(date, {
+      weekStartsOn: 1,
+      firstWeekContainsDate: 4,
+      // $ExpectedMistake
+      locale: {
+        options: { weekStartsOn: 0, firstWeekContainsDate: 1 }
+      }
+    })
+    assert.deepEqual(result, new Date(2005, 0 /* Jan */, 3, 0, 0, 0, 0))
+  })
+
+  it('throws `RangeError` if `options.weekStartsOn` is not convertable to 0, 1, ..., 6 or undefined', function() {
+    // $ExpectedMistake
+    var block = startOfWeekYear.bind(null, new Date(2007, 11 /* Dec */, 31), {
+      weekStartsOn: NaN
+    })
+    assert.throws(block, RangeError)
+  })
+
+  it('throws `RangeError` if `options.firstWeekContainsDate` is not convertable to 1, 2, ..., 7 or undefined', function() {
+    // $ExpectedMistake
+    var block = startOfWeekYear.bind(null, new Date(2007, 11 /* Dec */, 31), {
+      firstWeekContainsDate: NaN
+    })
+    assert.throws(block, RangeError)
+  })
+
+  it('throws TypeError exception if passed less than 1 argument', function() {
+    assert.throws(startOfWeekYear.bind(null), TypeError)
+  })
+})