Doing some tests with bitwise operations / shifting with JavaScript

0x80000000 >> 1 // returns -1073741824 (-0x40000000)

I would expect that to return 0x40000000 since

0x40000000 >> 1 // returns 0x20000000
0x20000000 >> 1 // returns 0x10000000

Doing some tests with bitwise operations / shifting with JavaScript

0x80000000 >> 1 // returns -1073741824 (-0x40000000)

I would expect that to return 0x40000000 since

0x40000000 >> 1 // returns 0x20000000
0x20000000 >> 1 // returns 0x10000000

• javascript
• bit-manipulation

• 3 That's because 0x800... is 0b10000..., and when you shift it right one place it bees 0b1100..., a negative number. On the other hand, 0x400... is 0b0100... and 0x200... is 0b0010... and they don't have this problem. – NullUserException Commented Dec 27, 2012 at 22:17
• @NullUserException sorry if this is very basic stuff, but why does it not bee 0b0100... ? can 0x80.. be shifted so it produces a positive 0x40.. ? – lostsource Commented Dec 27, 2012 at 22:23
• @NullUserException: Should be an answer – Lightness Races in Orbit Commented Dec 27, 2012 at 22:24
• @lostsource: Because there's nothing to "overwrite" the original value of that most-significant bit. Usually you'd apply a mask with & after performing a shift. – Lightness Races in Orbit Commented Dec 27, 2012 at 22:25
• thank you everyone for the information, I now understand the difference between an arithmetic and a logical shift – lostsource Commented Dec 27, 2012 at 22:29

2 Answers 2

Its an arithmetic shift that's why the sign is preserved, to do a logical shift use >>>

0x80000000 >>> 1 // returns 1073741824 (0x40000000)

In Javascript, >> is the Signed Right Shift Operator. In §11.7.2 of the language specification it says:

Performs a sign-filling bitwise right shift operation on the left operand by the amount specified by the right operand.

Before the shifting is done, the left operand is converted to a signed 32-bit integer (step 5 of the algorithm given in the specification). In your case this conversion turns the given positive number into a negative value. After that, the actual shifting is done, preserving the negative sign of the value.