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JavaScript: Is a Number a Power of Two (2)

See how to find a number with a base of 2 in C#.

Solution One: Use log₂

The simplest way to determine if a number is a power of two is to use log₂ and it will return an integer if so.

Solution Two: Use Brute Force

Simply keep dividing the number by 2 and then if the first bit is not turned on, and the number is not itself 1, then then the number is a power of 2 (base 2).

In other words, divide the number by two (2) iteratively until it becomes one (1). If the number becomes 1 then it is a power of 2 (base 2).

function isBase2 (num) {
	if (num == 0) {
		return false;
	}
	while (num != 1) {
		num /= 2;
		if ((num & 1) != 0 && num != 1) {
			return false;
		}
	}
	return true;
}
alert("0=" + isBase2(0) + " 1=" + isBase2(1) + " 2=" + isBase2(2) + " 3=" + isBase2(3) + " 4=" + isBase2(4) + " 10=" + isBase2(10) + " 12=" + isBase2(12) + " 16=" + isBase2(16));

Solution Three: Bit-wise Operations

Bit-wise operations are very fast and this should be the preferred method.

If the number is a power of two then the number AND (&) the same number minus one (1) will not be zero (0) unless the number itself is zero.

function isBase2 (num) {
	return (num & (num - 1)) == 0 && num != 0;
}
alert("0=" + isBase2(0) + " 1=" + isBase2(1) + " 2=" + isBase2(2) + " 3=" + isBase2(3) + " 4=" + isBase2(4) + " 10=" + isBase2(10) + " 12=" + isBase2(12) + " 16=" + isBase2(16));

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