JavaScript Faster multiplication or division by powers of 2


Example

Shifting bits left (right) is equivalent to multiplying (dividing) by 2. It's the same in base 10: if we "left-shift" 13 by 2 places, we get 1300, or 13 * (10 ** 2). And if we take 12345 and "right-shift" by 3 places and then remove the decimal part, we get 12, or Math.floor(12345 / (10 ** 3)). So if we want to multiply a variable by 2 ** n, we can just left-shift by n bits.

console.log(13 * (2 ** 6)) //13 * 64 = 832
console.log(13    <<   6)  //          832

Similarly, to do (floored) integer division by 2 ** n, we can right shift by n bits. Example:

console.log(1000 / (2 ** 4)) //1000 / 16 = 62.5
console.log(1000    >>   4)  //            62

It even works with negative numbers:

console.log(-80 / (2 ** 3)) //-80 / 8 = -10
console.log(-80      >> 3)  //          -10

In reality, speed of arithmetic is unlikely to significantly impact how long your code takes to run, unless you are doing on the order of 100s of millions of computations. But C programmers love this sort of thing!