javascript - What is the best way to determine if a given number is a power of two?

Question

I need to return true if the n is a power of two and false otherwise. It should go something like:

function PowerOfTwo(n){
  //code here
}

This is the way I currently do it :

function isPowerOfTwo(n){
  var x = Math.pow(2, Math.round(Math.log(n) / Math.log(2)));
  return x;
}

Are there any more effective ways ?

Answer 1

You can actually use ECMAScript5 Math.log:

function powerOfTwo(x) {
    return (Math.log(x)/Math.log(2)) % 1 === 0;
}

Remember, in math, to get a logarithm with an arbitrary base, you can just divide log₁₀ of the operand (x in this case) by log₁₀ of the base. And then to see if the number is a regular integer (and not a floating point), just check if the remainder is 0 by using the modulus % operator.

In ECMAScript6 you can do something like this:

function powerOfTwo(x) {
    return Math.log2(x) % 1 === 0;
}

See the MDN docs for Math.log2.