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JavaScript: The Definitive GuideJavaScript: The Definitive GuideSearch this book

5.8. Bitwise Operators

Despite the fact that all numbers in JavaScript are floating-point, the bitwise operators require numeric operands that have integer values. They operate on these integer operands using a 32-bit integer representation instead of the equivalent floating-point representation. Four of these operators perform Boolean algebra on the individual bits of the operands, behaving as if each bit in each operand were a boolean value and performing similar operations to those performed by the logical operators we saw earlier. The other three bitwise operators are used to shift bits left and right.

In JavaScript 1.0 and JavaScript 1.1, the bitwise operators return NaN if used with operands that are not integers or that are too large to fit in a 32-bit integer representation. JavaScript 1.2 and ECMAScript, however, simply coerce the operands to 32-bit integers by dropping any fractional part of the operand or any bits beyond the 32nd. The shift operators require a righthand operand between 0 and 31. After converting this operand to a 32-bit integer as described earlier, they drop any bits beyond the 5th, which yields a number in the appropriate range.

If you are not familiar with binary numbers and the binary representation of decimal integers, you can skip the operators described in this section. The purpose of these operators is not described here; they are needed for low-level manipulation of binary numbers and are not commonly used in JavaScript programming. The bitwise operators are:

Bitwise AND (&)
The & operator performs a Boolean AND operation on each bit of its integer arguments. A bit is set in the result only if the corresponding bit is set in both operands. For example, 0x1234 & 0x00FF evaluates to 0x0034.

Bitwise OR (|)
The | operator performs a Boolean OR operation on each bit of its integer arguments. A bit is set in the result if the corresponding bit is set in one or both of the operands. For example, 9 | 10 evaluates to 11.

Bitwise XOR (^)
The ^ operator performs a Boolean exclusive OR operation on each bit of its integer arguments. Exclusive OR means that either operand one is true or operand two is true, but not both. A bit is set in this operation's result if a corresponding bit is set in one (but not both) of the two operands. For example, 9 ^ 10 evaluates to 3.

Bitwise NOT (~)
The ~ operator is a unary operator that appears before its single integer argument. It operates by reversing all bits in the operand. Because of the way signed integers are represented in JavaScript, applying the ~ operator to a value is equivalent to changing its sign and subtracting 1. For example ~0x0f evaluates to 0xfffffff0, or -16.

Shift left (<<)
The << operator moves all bits in its first operand to the left by the number of places specified in the second operand, which should be an integer between 0 and 31. For example, in the operation a << 1, the first bit (the ones bit) of a becomes the second bit (the twos bit), the second bit of a becomes the third, etc. A zero is used for the new first bit, and the value of the 32nd bit is lost. Shifting a value left by one position is equivalent to multiplying by 2, shifting two positions is equivalent to multiplying by 4, etc. For example, 7 << 1 evaluates to 14.

Shift right with sign (>>)
The >> operator moves all bits in its first operand to the right by the number of places specified in the second operand (an integer between and 31). Bits that are shifted off the right are lost. The bits filled in on the left depend on the sign bit of the original operand, in order to preserve the sign of the result. If the first operand is positive, the result has zeros placed in the high bits; if the first operand is negative, the result has ones placed in the high bits. Shifting a value right one place is equivalent to dividing by 2 (discarding the remainder), shifting right two places is equivalent to integer division by 4, and so on. For example, 7 >> 1 evaluates to 3 and -7 >> 1 evaluates to -4.

Shift right with zero fill (>>>)
The >>> operator is just like the >> operator, except that the bits shifted in on the left are always zero, regardless of the sign of the first operand. For example, -1 >> 4 evaluates to -1, but -1 >>> 4 evaluates to 268435455 (0x0fffffff).



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