Ask questionsNaN byte representation mysteriously changes

  • Version: 13.12.0, 12.16.1, not 10.19.0
  • Platform: MacOS
  • Subsystem: v8

This is low priority and almost certainly a bug in v8, and I'm not sure if they care to fix it. But this program has mysterious behaviour:

const printBytes = (name, n) => {
  const bytes = Buffer.allocUnsafe(8)
  bytes.writeDoubleBE(n, 0)
  console.log(name + ': ', bytes)

const value = NaN

// Or equivalently:
// const b = Buffer.from('7ff8000000000000', 'hex')
// const value = new DataView(b.buffer).getFloat64(b.byteOffset, false)

const unused = {x: 5, value} // If you comment out this line, the problem goes away.
printBytes('same keys   ', ({x: 5, value}).value)
printBytes('diff keys   ', ({y: 5, value}).value)
printBytes('without prop', ({value}).value)
printBytes('naked       ', value)

Now, all those print statements should print the same value. But instead, this is what happens:

$ node test.js
same keys   :  <Buffer ff f7 ff ff ff f7 ff ff>
diff keys   :  <Buffer 7f f8 00 00 00 00 12 34>
without prop:  <Buffer 7f f8 00 00 00 00 12 34>
naked       :  <Buffer 7f f8 00 00 00 00 12 34>

The problem goes away if:

  • You comment out const unused = ...
  • You swap {x:5, value} to {value, x:5}
  • You use a value that isn't NaN



Answer questions targos

I'm not sure we can consider this a bug. There are multiple valid binary representations for NaN. I don't know if the ECMAScript spec requires only one of them to be materialized when the value is put in a typed array.


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