1 files changed, 46 insertions, 0 deletions
diff --git a/date-fns/src/startOfYear/test.ts b/date-fns/src/startOfYear/test.ts
new file mode 100644
index 0000000..bff3352
--- /dev/null
+++ b/date-fns/src/startOfYear/test.ts
@@ -0,0 +1,46 @@
+// @flow
+/* eslint-env mocha */
+
+import assert from 'power-assert'
+import startOfYear from '.'
+
+describe('startOfYear', function() {
+  it('returns the date with the time set to 00:00:00 and the date set to the first day of a year', function() {
+    const date = new Date(2014, 8 /* Sep */, 2, 11, 55, 0)
+    const result = startOfYear(date)
+    assert.deepEqual(result, new Date(2014, 0 /* Jan */, 1, 0, 0, 0, 0))
+  })
+
+  it('accepts a timestamp', function() {
+    const date = new Date(2014, 8 /* Sep */, 2, 11, 55, 0).getTime()
+    const result = startOfYear(date)
+    assert.deepEqual(result, new Date(2014, 0 /* Dec */, 1, 0, 0, 0, 0))
+  })
+
+  it('does not mutate the original date', function() {
+    const date = new Date(2014, 8 /* Sep */, 2, 11, 55, 0)
+    startOfYear(date)
+    assert.deepEqual(date, new Date(2014, 8 /* Sep */, 2, 11, 55, 0))
+  })
+
+  it('handles dates before 100 AD', function() {
+    const initialDate = new Date(0)
+    initialDate.setFullYear(9, 0 /* Jan */, 5)
+    initialDate.setHours(0, 0, 0, 0)
+    const expectedResult = new Date(0)
+    expectedResult.setFullYear(9, 0 /* Jan */, 1)
+    expectedResult.setHours(0, 0, 0, 0)
+    const result = startOfYear(initialDate)
+    assert.deepEqual(result, expectedResult)
+  })
+
+  it('returns `Invalid Date` if the given date is invalid', function() {
+    const result = startOfYear(new Date(NaN))
+    //@ts-expect-error
+    assert(result instanceof Date && isNaN(result))
+  })
+
+  it('throws TypeError exception if passed less than 1 argument', function() {
+    assert.throws(startOfYear.bind(null), TypeError)
+  })
+})