Checking power of 2 using bitwise operations in JavaScript

We are required to write a JavaScript function that takes in a number and determines whether or not it is a power of two.

For example −

f(23) = false
f(16) = true
f(1) = true
f(1024) = true

Approach −

Powers of two in binary form always have just one bit. Like this −

1: 0001
2: 0010
4: 0100
8: 1000

Therefore, after checking that the number is greater than zero, we can use a bitwise hack to test that one and only one bit is set. The same is shown below −

num & (num - 1)

Example

Following is the code −

const num1 = 256;
const num2 = 1024;
const isPowerOfTwo = (num = 1) => {
   if (num < 1) {
      return false;
   };
   return (num & (num - 1)) === 0;
};
console.log(isPowerOfTwo(num1));
console.log(isPowerOfTwo(num2));
console.log(isPowerOfTwo(1));
console.log(isPowerOfTwo(23));

Output

Following is the output on console −

true
true
true
false

AmitDiwan

Published on 11-Dec-2020 13:17:05