Sunday, February 3, 2013

Bitwise operations

Some time ago I was talking with a second-year student after a C exam. He was pretty satisfied about the job done, but asked me a question about the ~ operator. At first I didn't quite remember, so I told him that was probably the bitwise not. Later I discovered I was right, so I took the opportunity to refresh my knowledge of the bitwise operations in JavaScript. The purpose of this article is to show which bitwise operations JavaScript offers.

First of all, we can only apply bitwise operations on numeric operands that have integer values. These integer are represented in the operation as a 32-bit integer representation instead of the equivalent floating-point representation. Some of these operations perform Boolean algebra on the single bits, while the others are used to shift bits.

Bitwise NOT ( ~ )

This operator simply flips all the bits of the given integer. In numeric terms, it changes the sign of the number and subtract 1. For example ~0x0000000f will become 0xfffffff0 after the evaluation.

Bitwise AND ( & )

Performs a bitwise Boolean AND operation and set in the result the bit only if it is set in both operands. For example 0xfff666fff & 0x000fff000 will evaluate to 0x000666000.

Bitwise OR ( | )

Likewise the bitwise AND, the bitwise OR behaves in the same way, but a bit is set in the result only if it is set in one or both of the operands.

Bitwise XOR ( ^ )

The bitwise exclusive OR behaves like a normal XOR. The bit in the result is set only if the first operand is true and the second is false, or viceversa. If both are true or both are false, the bit in the result is set to 0.

Shift Right ( >> )

This operator moves all the bits in the first operand to the right by the number of bits specified in the second operand (up to 31). The bits that are shifter off the right are lost, and the filling is given by the initial sign of the first operand (if the operand is positive, it will be zero-filled, otherwise f-filled).

Shift Right with zero Fill ( >>> )

Behaves exactly like the shift right operator, but the filling on the left is always 0, regardless of the sign of the first operand at the beginning of the operation.

Shift Left ( << )

This operator moves all the bits in the first operand to the left by the number of bits specified in the second operand (again, up to 31). The rightmost part of the number is filled with 0s.

These operations are not really used by JavaScript programmers, but it's important to know they exist.

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