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+/* eslint-env mocha */
+
+import assert from 'power-assert'
+import differenceInISOWeekYears from '.'
+
+describe('differenceInISOWeekYears', function () {
+  it('returns the number of full ISO week-numbering years between the given dates', function () {
+    const result = differenceInISOWeekYears(
+      new Date(2012, 6 /* Jul */, 2, 18, 0),
+      new Date(2011, 6 /* Jul */, 2, 6, 0)
+    )
+    assert(result === 1)
+  })
+
+  it('returns a negative number if the time value of the first date is smaller', function () {
+    const result = differenceInISOWeekYears(
+      new Date(2011, 6 /* Jul */, 2, 6, 0),
+      new Date(2012, 6 /* Jul */, 2, 18, 0)
+    )
+    assert(result === -1)
+  })
+
+  it('accepts timestamps', function () {
+    const result = differenceInISOWeekYears(
+      new Date(2014, 6 /* Jul */, 2).getTime(),
+      new Date(2010, 6 /* Jul */, 2).getTime()
+    )
+    assert(result === 4)
+  })
+
+  it('handles dates before 100 AD', function () {
+    const firstDate = new Date(0)
+    firstDate.setFullYear(14, 0 /* Jan */, 1)
+    firstDate.setHours(0, 0, 0, 0)
+    const secondDate = new Date(0)
+    secondDate.setFullYear(0, 0 /* Jan */, 1)
+    secondDate.setHours(0, 0, 0, 0)
+    const result = differenceInISOWeekYears(firstDate, secondDate)
+    assert(result === 14)
+  })
+
+  describe('edge cases', function () {
+    it('the difference is less than an ISO year, but the given dates are in different calendar years', function () {
+      const result = differenceInISOWeekYears(
+        new Date(2012, 0 /* Jan */, 2),
+        new Date(2012, 0 /* Jan */, 1)
+      )
+      assert(result === 0)
+    })
+
+    it('the same for the swapped dates', function () {
+      const result = differenceInISOWeekYears(
+        new Date(2012, 0 /* Jan */, 1),
+        new Date(2012, 0 /* Jan */, 2)
+      )
+      assert(result === 0)
+    })
+
+    it('the ISO weeks and weekdays of the given dates are the same', function () {
+      const result = differenceInISOWeekYears(
+        new Date(2013, 11 /* Dec */, 30),
+        new Date(2012, 0 /* Jan */, 2)
+      )
+      assert(result === 2)
+    })
+
+    it('the given dates are the same', function () {
+      const result = differenceInISOWeekYears(
+        new Date(2014, 8 /* Sep */, 5, 0, 0),
+        new Date(2014, 8 /* Sep */, 5, 0, 0)
+      )
+      assert(result === 0)
+    })
+
+    it('does not return -0 when the given dates are the same', () => {
+      function isNegativeZero(x: number): boolean {
+        return x === 0 && 1 / x < 0
+      }
+
+      const result = differenceInISOWeekYears(
+        new Date(2014, 8 /* Sep */, 5, 0, 0),
+        new Date(2014, 8 /* Sep */, 5, 0, 0)
+      )
+
+      const resultIsNegative = isNegativeZero(result)
+      assert(resultIsNegative === false)
+    })
+  })
+
+  it('returns NaN if the first date is `Invalid Date`', function () {
+    const result = differenceInISOWeekYears(
+      new Date(NaN),
+      new Date(2017, 0 /* Jan */, 1)
+    )
+    assert(isNaN(result))
+  })
+
+  it('returns NaN if the second date is `Invalid Date`', function () {
+    const result = differenceInISOWeekYears(
+      new Date(2017, 0 /* Jan */, 1),
+      new Date(NaN)
+    )
+    assert(isNaN(result))
+  })
+
+  it('returns NaN if the both dates are `Invalid Date`', function () {
+    const result = differenceInISOWeekYears(new Date(NaN), new Date(NaN))
+    assert(isNaN(result))
+  })
+
+  it('throws TypeError exception if passed less than 2 arguments', function () {
+    assert.throws(differenceInISOWeekYears.bind(null), TypeError)
+    assert.throws(differenceInISOWeekYears.bind(null, 1), TypeError)
+  })
+})