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556 :: How does int64 work in JS?

• fuzzthink (orginal) [2014-09-12T06:51:30+02:00] view original

  JS's Number type is 64bit float. For integers, it is only precise up to 15 digits (~51 bits). When I enter y =

  9223372036854775807 (high of int64) directly in browser console, it (per spec) looses quite a bit of precision and gives me

  9223372036854776000. But when I compile "echo int64.high" to JS, it echoed 9223372036854775807, no precision is lost. I also tried arithmetics that results in > 15 digits, again, no precision is lost. How does this work?

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• Araq (orginal) [2014-09-12T09:26:19+02:00] view original

  I think it doesn't work at all and you happened to only test what is evaluated at compile time.

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• Jehan (orginal) [2014-09-12T13:39:31+02:00] view original

  In addition to what Araq says, JS represents all numbers internally as double precision floating point numbers. This means that you can represent integers up to 53 bits without loss of accuracy. The exception is that bit operations use only 32 bits. This can lead to interesting surprises, such as (running the following in node):

  
  > x = 1 << 30;
  1073741824
  > x & x;
  1073741824
  > x = x * 1024;
  1099511627776
  > x & x;
  0
  Run

  Working around that limitation is possible (that's what JS big integer libraries do), but is rarely worth the trouble.

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• Varriount (orginal) [2014-09-12T19:12:46+02:00] view original

  Perhaps Nimrod's compile time evaluation should emulate JavaScript's loss of precision (just kidding :3)

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