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Bitwise operators

The unary minus converted the string

“25”

into the number

–25

in the previous code (unary minus

also acts the same way as unary plus regarding hexadecimal and decimal values, but it also negates

the value).

Bitwise operators

The following set of operators work on numbers at their very base level, with the 32 bits that represent

them. Before examining these operators, I begin with a more detailed look into integers in ECMAScript.

Integers revisited

ECMAScript integers come in two specific flavors: signed (allowing both positive and negative values)

and unsigned (allowing only positive numbers). In ECMAScript, all integer literals are signed by

default. But what exactly does this mean?

Signed integers use the first 31 bits to represent the numeric value of the integer, whereas the 32nd bit

represents the sign of the number, 0 for positive or 1 for negative. The number values can range from

–2147483648 to 2147483647.

You can store signed integers in binary form in two different ways, one for positive numbers and one for

negative numbers. Positive numbers are stored in true binary format, with each of the first 31 bits repre-

senting a power of 2, starting with the first bit (called bit 0), which represents 2

; the second bit (bit 1)

represents 2

, and so on. If any bits are unused, they are filled with 0s and essentially ignored. For exam-

ple, the number 18 is represented as shown in Figure 2-4.

Figure 2-4

The binary version of 18 uses only the first five bits, which are the significant bits for this number. When

converting a number into a binary string (as discussed earlier), you see only the significant bits:

var iNum = 18;

alert(iNum.toString(2)); //outputs “10010”

This code outputs only

“10010”

instead of the whole 32-bit representation. The other bits really aren’t

important because using just these five bits makes possible to determine the decimal value (Figure 2-5).

00000000000000000000000000010010

The number 18

Bit 31

Bit 0

Filler

ECMAScript Basics

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