Fix negative zero bug in differenceInX functions (closes #692) (#739)

src/differenceInCalendarDays/test.js

@@ -69,6 +69,20 @@ describe('differenceInCalendarDays', function () {
       )
       assert(result === 0)
     })
+
+    it('does not return -0 when the given dates are the same', () => {
+      function isNegativeZero (x) {
+        return x === 0 && (1 / x < 0)
+      }
+
+      var result = differenceInCalendarDays(
+        new Date(2014, 8 /* Sep */, 5, 0, 0),
+        new Date(2014, 8 /* Sep */, 5, 0, 0)
+      )
+
+      var resultIsNegative = isNegativeZero(result)
+      assert(resultIsNegative === false)
+    })
   })
 
   it('returns NaN if the first date is `Invalid Date`', function () {

src/differenceInCalendarISOWeekYears/test.js

@@ -80,6 +80,20 @@ describe('differenceInCalendarISOWeekYears', function () {
       )
       assert(result === 0)
     })
+
+    it('does not return -0 when the given dates are the same', () => {
+      function isNegativeZero (x) {
+        return x === 0 && (1 / x < 0)
+      }
+
+      var result = differenceInCalendarISOWeekYears(
+        new Date(2014, 8 /* Sep */, 5, 0, 0),
+        new Date(2014, 8 /* Sep */, 5, 0, 0)
+      )
+
+      var resultIsNegative = isNegativeZero(result)
+      assert(resultIsNegative === false)
+    })
   })
 
   it('returns NaN if the first date is `Invalid Date`', function () {

src/differenceInCalendarISOWeeks/test.js

@@ -69,6 +69,20 @@ describe('differenceInCalendarISOWeeks', function () {
       )
       assert(result === 0)
     })
+
+    it('does not return -0 when the given dates are the same', () => {
+      function isNegativeZero (x) {
+        return x === 0 && (1 / x < 0)
+      }
+
+      var result = differenceInCalendarISOWeeks(
+        new Date(2014, 8 /* Sep */, 5, 0, 0),
+        new Date(2014, 8 /* Sep */, 5, 0, 0)
+      )
+
+      var resultIsNegative = isNegativeZero(result)
+      assert(resultIsNegative === false)
+    })
   })
 
   it('returns NaN if the first date is `Invalid Date`', function () {

src/differenceInCalendarMonths/test.js

@@ -69,6 +69,20 @@ describe('differenceInCalendarMonths', function () {
       )
       assert(result === 0)
     })
+
+    it('does not return -0 when the given dates are the same', () => {
+      function isNegativeZero (x) {
+        return x === 0 && (1 / x < 0)
+      }
+
+      var result = differenceInCalendarMonths(
+        new Date(2014, 8 /* Sep */, 5, 0, 0),
+        new Date(2014, 8 /* Sep */, 5, 0, 0)
+      )
+
+      var resultIsNegative = isNegativeZero(result)
+      assert(resultIsNegative === false)
+    })
   })
 
   it('returns NaN if the first date is `Invalid Date`', function () {

src/differenceInCalendarQuarters/test.js

@@ -69,6 +69,20 @@ describe('differenceInCalendarQuarters', function () {
       )
       assert(result === 0)
     })
+
+    it('does not return -0 when the given dates are the same', () => {
+      function isNegativeZero (x) {
+        return x === 0 && (1 / x < 0)
+      }
+
+      var result = differenceInCalendarQuarters(
+        new Date(2014, 8 /* Sep */, 5, 0, 0),
+        new Date(2014, 8 /* Sep */, 5, 0, 0)
+      )
+
+      var resultIsNegative = isNegativeZero(result)
+      assert(resultIsNegative === false)
+    })
   })
 
   it('returns NaN if the first date is `Invalid Date`', function () {

src/differenceInCalendarWeeks/test.js

@@ -117,6 +117,20 @@ describe('differenceInCalendarWeeks', function () {
       )
       assert(result === 0)
     })
+
+    it('does not return -0 when the given dates are the same', () => {
+      function isNegativeZero (x) {
+        return x === 0 && (1 / x < 0)
+      }
+
+      var result = differenceInCalendarWeeks(
+        new Date(2014, 8 /* Sep */, 5, 0, 0),
+        new Date(2014, 8 /* Sep */, 5, 0, 0)
+      )
+
+      var resultIsNegative = isNegativeZero(result)
+      assert(resultIsNegative === false)
+    })
   })
 
   it('returns NaN if the first date is `Invalid Date`', function () {

src/differenceInCalendarYears/test.js

@@ -69,6 +69,20 @@ describe('differenceInCalendarYears', function () {
       )
       assert(result === 0)
     })
+
+    it('does not return -0 when the given dates are the same', () => {
+      function isNegativeZero (x) {
+        return x === 0 && (1 / x < 0)
+      }
+
+      var result = differenceInCalendarYears(
+        new Date(2014, 8 /* Sep */, 5, 0, 0),
+        new Date(2014, 8 /* Sep */, 5, 0, 0)
+      )
+
+      var resultIsNegative = isNegativeZero(result)
+      assert(resultIsNegative === false)
+    })
   })
 
   it('returns NaN if the first date is `Invalid Date`', function () {

src/differenceInDays/index.js

@@ -45,13 +45,12 @@ export default function differenceInDays (dirtyDateLeft, dirtyDateRight, dirtyOp
   var sign = compareAsc(dateLeft, dateRight, dirtyOptions)
   var difference = Math.abs(differenceInCalendarDays(dateLeft, dateRight, dirtyOptions))
 
-  // exit early if the days are the same
-  if (difference === 0) return 0
-
   dateLeft.setDate(dateLeft.getDate() - sign * difference)
 
   // Math.abs(diff in full days - diff in calendar days) === 1 if last calendar day is not full
   // If so, result must be decreased by 1 in absolute value
   var isLastDayNotFull = compareAsc(dateLeft, dateRight, dirtyOptions) === -sign
-  return sign * (difference - isLastDayNotFull)
+  var result = sign * (difference - isLastDayNotFull)
+  // Prevent negative zero
+  return result === 0 ? 0 : result
 }

src/differenceInDays/test.js

@@ -70,18 +70,17 @@ describe('differenceInDays', function () {
       assert(result === 0)
     })
 
-    it('does not return -0 when given dates are the same', () => {
-      // thank you Chris West! @ChrisWestBlog, http://cwestblog.com/2014/02/25/javascript-testing-for-negative-zero/
-      function isNegative (n) {
-        return ((n = +n) || 1 / n) < 0
+    it('does not return -0 when the given dates are the same', () => {
+      function isNegativeZero (x) {
+        return x === 0 && (1 / x < 0)
       }
 
       var result = differenceInDays(
         new Date(2014, 8 /* Sep */, 5, 0, 0),
         new Date(2014, 8 /* Sep */, 5, 0, 0)
       )
 
-      var resultIsNegative = isNegative(result)
+      var resultIsNegative = isNegativeZero(result)
       assert(resultIsNegative === false)
     })
   })

src/differenceInHours/test.js

@@ -69,6 +69,20 @@ describe('differenceInHours', function () {
       )
       assert(result === 0)
     })
+
+    it('does not return -0 when the given dates are the same', () => {
+      function isNegativeZero (x) {
+        return x === 0 && (1 / x < 0)
+      }
+
+      var result = differenceInHours(
+        new Date(2014, 8 /* Sep */, 5, 0, 0),
+        new Date(2014, 8 /* Sep */, 5, 0, 0)
+      )
+
+      var resultIsNegative = isNegativeZero(result)
+      assert(resultIsNegative === false)
+    })
   })
 
   it('returns NaN if the first date is `Invalid Date`', function () {

src/differenceInISOWeekYears/index.js

@@ -45,5 +45,7 @@ export default function differenceInISOWeekYears (dirtyDateLeft, dirtyDateRight,
   // if last calendar ISO year is not full
   // If so, result must be decreased by 1 in absolute value
   var isLastISOWeekYearNotFull = compareAsc(dateLeft, dateRight, dirtyOptions) === -sign
-  return sign * (difference - isLastISOWeekYearNotFull)
+  var result = sign * (difference - isLastISOWeekYearNotFull)
+  // Prevent negative zero
+  return result === 0 ? 0 : result
 }

src/differenceInISOWeekYears/test.js

@@ -80,6 +80,20 @@ describe('differenceInISOWeekYears', function () {
       )
       assert(result === 0)
     })
+
+    it('does not return -0 when the given dates are the same', () => {
+      function isNegativeZero (x) {
+        return x === 0 && (1 / x < 0)
+      }
+
+      var result = differenceInISOWeekYears(
+        new Date(2014, 8 /* Sep */, 5, 0, 0),
+        new Date(2014, 8 /* Sep */, 5, 0, 0)
+      )
+
+      var resultIsNegative = isNegativeZero(result)
+      assert(resultIsNegative === false)
+    })
   })
 
   it('returns NaN if the first date is `Invalid Date`', function () {

src/differenceInMilliseconds/test.js

@@ -37,6 +37,20 @@ describe('differenceInMilliseconds', function () {
     assert(result === 0)
   })
 
+  it('does not return -0 when the given dates are the same', () => {
+    function isNegativeZero (x) {
+      return x === 0 && (1 / x < 0)
+    }
+
+    var result = differenceInMilliseconds(
+      new Date(2014, 8 /* Sep */, 5, 0, 0),
+      new Date(2014, 8 /* Sep */, 5, 0, 0)
+    )
+
+    var resultIsNegative = isNegativeZero(result)
+    assert(resultIsNegative === false)
+  })
+
   it('returns NaN if the first date is `Invalid Date`', function () {
     var result = differenceInMilliseconds(
       new Date(NaN),

src/differenceInMinutes/test.js

@@ -69,6 +69,20 @@ describe('differenceInMinutes', function () {
       )
       assert(result === 0)
     })
+
+    it('does not return -0 when the given dates are the same', () => {
+      function isNegativeZero (x) {
+        return x === 0 && (1 / x < 0)
+      }
+
+      var result = differenceInMinutes(
+        new Date(2014, 8 /* Sep */, 5, 0, 0),
+        new Date(2014, 8 /* Sep */, 5, 0, 0)
+      )
+
+      var resultIsNegative = isNegativeZero(result)
+      assert(resultIsNegative === false)
+    })
   })
 
   it('returns NaN if the first date is `Invalid Date`', function () {

src/differenceInMonths/index.js

@@ -41,5 +41,7 @@ export default function differenceInMonths (dirtyDateLeft, dirtyDateRight, dirty
   // Math.abs(diff in full months - diff in calendar months) === 1 if last calendar month is not full
   // If so, result must be decreased by 1 in absolute value
   var isLastMonthNotFull = compareAsc(dateLeft, dateRight, dirtyOptions) === -sign
-  return sign * (difference - isLastMonthNotFull)
+  var result = sign * (difference - isLastMonthNotFull)
+  // Prevent negative zero
+  return result === 0 ? 0 : result
 }

src/differenceInMonths/test.js

@@ -69,6 +69,20 @@ describe('differenceInMonths', function () {
       )
       assert(result === 0)
     })
+
+    it('does not return -0 when the given dates are the same', () => {
+      function isNegativeZero (x) {
+        return x === 0 && (1 / x < 0)
+      }
+
+      var result = differenceInMonths(
+        new Date(2014, 8 /* Sep */, 5, 0, 0),
+        new Date(2014, 8 /* Sep */, 5, 0, 0)
+      )
+
+      var resultIsNegative = isNegativeZero(result)
+      assert(resultIsNegative === false)
+    })
   })
 
   it('returns NaN if the first date is `Invalid Date`', function () {

src/differenceInQuarters/test.js

@@ -69,6 +69,20 @@ describe('differenceInQuarters', function () {
       )
       assert(result === 0)
     })
+
+    it('does not return -0 when the given dates are the same', () => {
+      function isNegativeZero (x) {
+        return x === 0 && (1 / x < 0)
+      }
+
+      var result = differenceInQuarters(
+        new Date(2014, 8 /* Sep */, 5, 0, 0),
+        new Date(2014, 8 /* Sep */, 5, 0, 0)
+      )
+
+      var resultIsNegative = isNegativeZero(result)
+      assert(resultIsNegative === false)
+    })
   })
 
   it('returns NaN if the first date is `Invalid Date`', function () {

src/differenceInSeconds/test.js

@@ -69,6 +69,20 @@ describe('differenceInSeconds', function () {
       )
       assert(result === 0)
     })
+
+    it('does not return -0 when the given dates are the same', () => {
+      function isNegativeZero (x) {
+        return x === 0 && (1 / x < 0)
+      }
+
+      var result = differenceInSeconds(
+        new Date(2014, 8 /* Sep */, 5, 0, 0),
+        new Date(2014, 8 /* Sep */, 5, 0, 0)
+      )
+
+      var resultIsNegative = isNegativeZero(result)
+      assert(resultIsNegative === false)
+    })
   })
 
   it('returns NaN if the first date is `Invalid Date`', function () {

src/differenceInWeeks/test.js

@@ -69,6 +69,20 @@ describe('differenceInWeeks', function () {
       )
       assert(result === 0)
     })
+
+    it('does not return -0 when the given dates are the same', () => {
+      function isNegativeZero (x) {
+        return x === 0 && (1 / x < 0)
+      }
+
+      var result = differenceInWeeks(
+        new Date(2014, 8 /* Sep */, 5, 0, 0),
+        new Date(2014, 8 /* Sep */, 5, 0, 0)
+      )
+
+      var resultIsNegative = isNegativeZero(result)
+      assert(resultIsNegative === false)
+    })
   })
 
   it('returns NaN if the first date is `Invalid Date`', function () {

src/differenceInYears/index.js

@@ -41,5 +41,7 @@ export default function differenceInYears (dirtyDateLeft, dirtyDateRight, dirtyO
   // Math.abs(diff in full years - diff in calendar years) === 1 if last calendar year is not full
   // If so, result must be decreased by 1 in absolute value
   var isLastYearNotFull = compareAsc(dateLeft, dateRight, dirtyOptions) === -sign
-  return sign * (difference - isLastYearNotFull)
+  var result = sign * (difference - isLastYearNotFull)
+  // Prevent negative zero
+  return result === 0 ? 0 : result
 }