Double bitwise NOT (~~) – James Padolsey

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Who would have thought that JavaScript’s rare and mysterious bitwise operators could be so useful? I first discovered this trick a while ago from a slideshow on JavaScript performance, by Thomas Fuchs. The “trick” is as follows:

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The bitwise NOT operator (~) will take its operand, convert it to a 32-bit integer, and will invert each bit so that each 0 becomes a 1 and vice versa.

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00000000000000000000000000001001 ...becomes 11111111111111111111111111110110

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The effect of this, given the expression ~foo is -(foo + 1). A double bitwise NOT operation (~~foo) will therefore result in -(-(foo + 1) + 1). This only remains true for integers though; given all the potential operands, the real equivalent expression to ~~ is probably something like:

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typeof foo === 'number' && !isNaN(foo) && foo !== Infinity ? foo > 0 ? Math.floor(foo) : Math.ceil(foo) : 0; // This is ONLY effectively the same... this is // NOT what happens internally!

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This is obviously a fair bit slower than ~~.

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If the operand is a number and it’s not NaN or Infinity then ~~ will have the effect of rounding it towards zero (Math.ceil for negative, Math.floor for positive). If it’s not a number, then I believe the internal ToInt32 function converts it to zero.

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Here are some examples of the double-bitwise NOT operation in all its glory:

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~~null; // => 0 ~~undefined; // => 0 ~~0; // => 0 ~~{}; // => 0 ~~[]; // => 0 ~~(1/0); // => 0 ~~false; // => 0 ~~true; // => 1 ~~1.2543; // => 1 ~~4.9; // => 4 ~~(-2.999); // => -2

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~~‘s flooring capabilities make it a better alternative to Math.floor if you know you’re dealing with positives — it’s faster and takes up less characters. It’s not quite as readable though, but I’m hoping that ~~ will slowly become a very well-known technique in the JS arena, so we can all use it without fearing accusations.

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It’s quite useful for normalizing arguments that you expect to be integers too. Take this example from what the MDC recommends for providing Array.prototype.indexOf to non-supporting browsers:

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/Array.prototype.indexOf = function.../ var from = Number(arguments[1]) || 0;
from = (from < 0)
? Math.ceil(from)
: Math.floor(from);

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In goes ~~:

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/Array.prototype.indexOf = function.../ var from = ~~arguments[1];

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Go forth and double-NOT!

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Thanks for reading! Please share your thoughts with me on Twitter. Have a great day!

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