CODEWITHSUNDEEP
javascriptarithmetic-expressions
anu
anu

Reputation: 1007

Unexpected Right Shift Operator behavior

Please look at the following Node.js REPL output:

> n=10
10
> console.log(n.toString(2)); //ok ?
1010
undefined
> m=n>>1 
5
> console.log(m.toString(2)); //cool :)
101

This is working as expected. Now look at this:

> n=Number.MAX_VALUE
1.7976931348623157e+308
> m=n>>1
0
> n.toString(16) // to read easier
'fffffffffffff800000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000'
> m.toString(16) // to read easier
'0'

What just happened here? Shouldn't me right shifting

fffffffffffff800000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

by 1 digit give me

7ffffffffffffc00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000

How did I get a zero instead?

Also, I noticed that it is not just V8/Node issue. I get the same result in Firebug.

Upvotes: 0

Views: 49

Answers (2)

Alnitak
Alnitak

Reputation: 339916

Javascript's bit-level operations only work on 32-bit signed values (or unsigned in the case of the zero-extending >>> operator). Anything above those 32 bits is set to zero before the bit operation takes place.

Upvotes: 2

Nayuki
Nayuki

Reputation: 18542

In JavaScript when you compute x >> y, x is converted into a signed 32-bit integer. Then it is shifted right by y places.

Your value 0xfffffffffffff800000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000 is converted into 0x00000000 because those are the lowest 32 bits. Right shifting this gives zero.

Upvotes: 1

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