Converting a double to an int in Javascript without rounding

Question

In C  the following code returns 2   double d   2 9  int i    int d  Debug WriteLine i     In Javascript  however  the only way of converting a  double  to an  int  that I m aware of is by using Math round floor toFixed etc  Is there a way of converting to an int in Javascript without rounding  I m aware of the performance implications of Number   so I d rather avoid converting it to a string if at all possible

User · Answer

Just use parseInt   and be sure to include the radix so you get predictable results   parseInt d  10

User · Answer

A trick to truncate that avoids a function call entirely is var number   2 9 var truncated   number - number   1  console log truncated      2   To round a floating-point number to the nearest integer  use the addition subtraction trick  This works for numbers with absolute value  lt  2   51  var number   2 9 var rounded   number   6755399441055744 0 - 6755399441055744 0       2 52   2 51  console log rounded      3   Note  Halfway values are rounded to the nearest even using  quot round half to even quot  as the tie-breaking rule  Thus  for example   23 5 becomes  24  as does  24 5  This variant of the round-to-nearest mode is also called bankers  rounding  The magic number 6755399441055744 0 is explained in the stackoverflow post  quot A fast method to round a double to a 32-bit int explained quot    x000D   x000D     Round to whole integers using arithmetic operators let trunc    v    gt  v - v   1  let ceil     v    gt  trunc v   1  gt  0   v   1   v   let floor    v    gt  trunc v   1  lt  0   v - 1   v   let round    v    gt  trunc v  lt  0   v - 0 5   v   0 5    let roundHalfEven    v    gt  v   6755399441055744 0 - 6755399441055744 0      2 52   2 51   console log  number  floor   ceil  round  trunc    var array    1 5  1 4  1 0  -1 0  -1 4  -1 5   array forEach x   gt        let f   x   gt   x  toString   padStart 6           console log    f x     f floor x      f ceil x      f round x      f trunc x             x000D   x000D   x000D

User · Answer

I find the  parseInt  suggestions to be pretty curious  because  parseInt  operates on strings by design  That s why its name has the word  parse  in it   A trick that avoids a function call entirely is  var truncated     number    The double application of the     unary operator will leave you with a truncated version of a double-precision value  However  the value is limited to 32 bit precision  as with all the other JavaScript operations that implicitly involve considering numbers to be integers  like array indexing and the bitwise operators    edit  mdash  In an update quite a while later  another alternative to the    trick is to bitwise-OR the value with zero   var truncated   number 0

User · Answer

Use parseInt     var num   2 9 console log parseInt num  10       2   You can also use     var num   2 9 console log num   0      2

User · Answer

Similar to C  casting to  int  with just using standard lib  Math trunc 1 6     1 Math trunc -1 6     -1

User · Answer

There is no such thing as an int in Javascript  All Numbers are actually doubles behind the scenes  so you can t rely on the type system to issue a rounding order for you as you can in C or C    You don t need to worry about precision issues  since doubles correctly represent any integer up to 2 53  but you really are stuck with using Math floor  or other equivalent tricks  if you want to round to the nearest integer      Most JS engines use native ints when they can but all in all JS numbers must still have double semantics

User · Answer

As  Quentin and  Pointy pointed out in their comments  it s not a good idea to use parseInt   because it is designed to convert a string to an integer  When you pass a decimal number to it  it first converts the number to a string  then casts it to an integer  I suggest you use Math trunc    Math floor      num    v   num   0  num  lt  lt  0  or num  gt  gt  0 depending on your needs  This performance test demonstrates the difference in parseInt   and Math floor   performance  Also  this post explains the difference between the proposed methods

[javascript] Converting a double to an int in Javascript without rounding

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