Testing whether a value is odd or even

Question

I decided to create simple isEven and isOdd function with a very simple algorithm   function isEven n      n   Number n     return n     0       n  amp  amp    n 2       function isOdd n      return isEven Number n    1       That is OK if n is with certain parameters  but fails for many scenarios  So I set out to create robust functions that deliver correct results for as many scenarios as I could  so that only integers within the limits of javascript numbers are tested  everything else returns false  including   and - infinity   Note that zero is even      Returns true if           n is an integer that is evenly divisible by 2       Zero    -0  is even    Returns false if n is not an integer  not even or NaN    Guard against empty string   function  global       function basicTests n            Deal with empty string     if  n                return false          Convert n to Number  may set to NaN      n   Number n           Deal with NaN     if  isNaN n          return false          Deal with infinity -      if  n     Number NEGATIVE INFINITY    n     Number POSITIVE INFINITY        return false          Return n as a number     return n         function isEven n            Do basic tests     if  basicTests n      false        return false          Convert to Number and proceed     n   Number n           Return true false     return n     0       n  amp  amp    n 2          global isEven   isEven        Returns true if n is an integer and  n 1  is even      Returns false if n is not an integer or  n 1  is not even      Empty string evaluates to zero so returns false  zero is even    function isOdd n            Do basic tests     if  basicTests n      false        return false          Return true false     return n     0       n  amp  amp   n 2          global isOdd   isOdd     this      Can anyone see any issues with the above  Is there a better  i e  more accurate  faster or more concise without being obfuscated  version   There are various posts relating to other languages  but I can t seem to find a definitive version for ECMAScript

User · Answer

We just need one line of code for this   Here a newer and alternative way to do this  using the new ES6 syntax for JS functions  and the one-line syntax for the if-else statement call   const isEven   num   gt    num   2     0    true   false   alert isEven 8       true alert isEven 9       false alert isEven -8      true

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A simple modification improvement of Steve Mayne answer   function isEvenOrOdd n       if n     parseFloat n            return isNumber n   amp  amp   n   2    0             return false      Note  Returns false if invalid

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var isOdd   x   gt  Boolean x   2   var isEven   x   gt   isOdd x

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x000D   x000D  for var a 0  a lt  20 a        if a 2   0          console log  Odd number   a           for var b 0  b lt  20 a         if b 2   0           console log  Even number   b                   x000D   x000D   x000D

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function isEven n   return parseInt n  2   0 true parseInt n    0 true false    when 0 even wanted but  isEven 0    true isEven 1    false isEven 2    true isEven 142856    true isEven 142856 142857   true isEven 142857 1457   false

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var num   someNumber     isEven  parseInt num 2      num 2   isEven   true   isEven   false

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Note  there are also negative numbers   function isOddInteger n       return isInteger n   amp  amp   n   2     0       where  function isInteger n       return n     parseInt n  10

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var isEven   function number           Your code goes here      if  number   2    0          return true             else         return false

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Different way   var isEven   function number         Your code goes here    if    number 2  - Math floor number 2       0   return true   else  return false        isEven 69

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Otherway using strings because why not  function isEven   num       return String   num 2  indexOf          -1

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Use modulus   function isEven n       return n   2    0     function isOdd n       return Math abs n   2     1      You can check that any value in Javascript can be coerced to a number with   Number isFinite parseFloat n     This check should preferably be done outside the isEven and isOdd functions  so you don t have to duplicate error handling in both functions

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To test whether or not you have a odd or even number  this also works     const comapare   x   gt  integer checkNumber x     function checkNumber  x       if  x   2    0           return true           else if  x   2    0           return false           function integer  x       if  x           console log  even             else          console log  odd

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To complete Robert Brisita s bit test    if    i  amp  1            Even

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This one is more simple     var num   3   instead get your value here   var aa     Even    Odd       alert aa num   2

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if  i   2    return odd numbers    if  i   2 - 1    return even numbers

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A few  x   2    0     Check if even    x  amp  1      bitmask the value with 1 then invert     x  gt  gt  1   lt  lt  1     x     divide value by 2 then multiply again and check against original value   x amp 1     flip the bits and bitmask

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I prefer using a bit test   if i  amp  1           ODD   else          EVEN     This tests whether the first bit is on which signifies an odd number

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if  testNum    0   else if  testNum   2     0   else if   testNum   2     0

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How about the following  I only tested this in IE  but it was quite happy to handle strings representing numbers of any length  actual numbers that were integers or floats  and both functions returned false when passed a boolean  undefined  null  an array or an object   Up to you whether you want to ignore leading or trailing blanks when a string is passed in - I ve assumed they are not ignored and cause both functions to return false    function isEven n       return   -  d  02468    test n      function isOdd n       return   -  d  13579    test n

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Maybe this  if ourNumber   2     0

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Why not just do this       function oddOrEven num           if num   2    0              return  even           return  odd             oddOrEven num

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Using modern javascript style   const NUMBERS    nul one two three four five six seven ocho nueve  split       const isOdd    n  gt  NUMBERS n   10  indexOf  e    -1 const isEven   n  gt  isOdd  n 1

[javascript] Testing whether a value is odd or even

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