![\"Not](\"http:\/\/bateru.com\/news\/wp-content\/uploads\/2011\/03\/NOT-GATE-300x129.png\")
Not gate<\/span><\/div>
\nA weird part of javascript is trying to toggle the bits in a number. For most programming languages, like C and Java, ~ represents a bitwise NOT operation, aka one’s complement. But in javascript, ~ produces the two’s complement. Since ~num gives the two’s complement, equivalent to -(num + 1), then one would suspect that ~(num -1) should give back the one’s complement.
\nHowever that’s not the case, as the next example demonstrates.<\/p>\n
\r\nvar num = 9;\r\nnum.toString(2); \/\/returns 1001\r\n(~num).toString(2);\t\/\/ returns \"-1010\"\r\n~(num - 1).toString(2); \/\/returns \"-1001\"\r\n<\/pre>\nAs you can see from the example, there are two main problems. The first is that the sign bit is replaced with a negative sign. And second, the bits don’t get toggled.
\nTo fix this problem most programmers would just XOR the number with a mask to toggle the bits. Although this works, it quickly produces the unintended result once you XOR a number larger than the mask.
\nExample:<\/b><\/p>\n\r\nvar BIT_MASK = 0xF;\r\n(9).toString(2);\t\t\t\/\/returns \"1001\"\r\n(9 ^ BIT_MASK).toString(2);\t\t\/\/returns \"110\"\r\n(30).toString(2);\t\t\t\/\/returns \"11110\"\r\n(30 ^ BIT_MASK).toString(2);\t\t\/\/returns \"10001\"\r\n<\/pre>\nA simple solution to this problem is to not to use javascript for bitwise operations.
\nHowever, if javascript is required then use the following code to implement bitwise NOT operation.
\nMethod 1:<\/b> Simple approach.<\/p>\n\r\nvar toggleBits = function( dec ){\r\n var mask = 0xFFFFFFF;\r\n return ( dec ^ mask ).toString(2).match( RegExp( \"[01]{\"+(dec.toString(2).length)+\"}$\" ) )[0];\r\n};\r\n\r\nvar num = 23;\r\nnum.toString(2);\t\t\/\/ returns 10111\r\nnum = toggleBits(num);\t \/\/ num = 01000;\r\nnum = parseInt( num, 2);\t\/\/ num = 8, binary = 1000\r\n<\/pre>\nMethod 2:<\/b> More advance features.<\/p>\n\r\n\/\/ Programer: Larry Battle\r\n\/\/ Purpose: Provide a bit toggle function for javascript.\r\nvar getStrCopy = function (str, copies) {\r\n\tvar newStr = str;\r\n\tcopies = (copies > 0) ? copies : 1;\r\n\twhile (--copies) {\r\n\t\tnewStr += str;\r\n\t}\r\n\treturn newStr;\r\n};\r\nvar convertDecToBase = function ( dec, base, length, padding ) {\r\n\tpadding = padding || '0' ;\r\n\tvar num = dec.toString( base );\r\n\tlength = length || num.length;\r\n\tif (num.length !== length) {\r\n\t\tif (num.length > length) {\r\n\t\t\tthrow new Error(\"convertDecToBase(): num(\" + num + \") > length(\" + length + \") too long.\");\r\n\t\t}\r\n\t\tnum = getStrCopy( padding, (length - num.length)) + num;\r\n\t}\r\n\treturn num;\r\n};\r\nvar formatBinaryStr = function( str ){\r\n return str.replace( \/\\d{4}\/g, '$& ' ).replace( \/\\s$\/,'');\r\n};\r\nvar toggleBits = function( dec, length, doFormat ){\r\n var str = convertDecToBase( dec, 2, length || 8 );\r\n var binaryStr = str.replace( \/0\/g, 'o' ).replace( \/1\/g, '0').replace( \/o\/g, '1' );\r\n return ( doFormat ) ? formatBinaryStr( binaryStr ) : binaryStr ;\r\n};\r\n\r\n\/\/ The following requires Firebug or Google Chrome Dev Tools\r\nclear();\r\n\r\n\/\/ Test case\r\nvar num = 230;\r\nvar testNum = parseInt( toggleBits(num), 2 );\r\ntestNum = parseInt( toggleBits( testNum ), 2 );\r\nconsole.log( \"(Test Case) 2 Bit Toggles will give the original number: \" + (testNum == num) );\r\n\r\n\/\/ Examples: \r\nconsole.log( toggleBits( 1 ) ); \/\/ displays \"11111110\"\r\nconsole.log( toggleBits( 2 ) ); \/\/ displays \"11111101\"\r\nconsole.log( toggleBits( 50, 16 ) );\/\/ displays \"1111111111001101\"\r\nconsole.log( toggleBits( 15, 8, true ) ); \/\/ displays \"1111 0000\"\r\nconsole.log( toggleBits( 520, 16, true ) ); \/\/ displays \"1111 1101 1111 0111\"\r\n<\/pre>\n\nFor more information check out Mozilla’s Bitwise Operators Reference<\/a><\/p>\n