Given two numbers, A and B. Write a program to count the number of bits needed to be flipped to obtain B from A. 

Examples: 

Input: A = 10, B = 20
Output: 4
Explanation: Binary representation of A is 00001010
Binary representation of B is 00010100
We need to flip highlighted four bits in A to obtain B from it.

Input: A = 7, B = 10
Output: 3
Explanation: Binary representation of A is 00000111
Binary representation of B is 00001010
We need to flip highlighted three bits in A to obtain B from it.

Below are the approaches to Count Number of Bits to be Flipped to Convert A to B using JavaScript:

Using the XOR operator

We can use the XOR operator (^) to find the bits that are different between A and B. Then, we count the number of set bits (bits with value 1) in the result.

Example: Implementation of a program to find the Number of bits to be flipped to obtain B from A using the XOR operator

function countBitsToFlipXOR(A, B) {

   // Find the bits that are different between A and B
    let xorResult = A ^ B; 
    let count = 0;
    
    // Count the number of set bits in the XOR result
    while (xorResult > 0) {
    
    // Check if the least significant bit is 1
        count += xorResult & 1; 
        xorResult >>= 1; // Right shift to check the next bit
    }
    return count;
}

const A1 = 10, B1 = 20;
console.log("Using XOR Operator:", countBitsToFlipXOR(A1, B1)); 

Using XOR Operator: 4

Time Complexity: O(log(max(A, B)))

Auxiliary Space: O(1)

Using the AND operator

We first Iterate through each bit of numbers A and B and than use AND operator to extract the least significant bit of A and B.If the bits are different, increment the flips count, and then right shift A and B to check the next bit.

Example: Implementation of program to find Number of bits to flipped to obtain B from A using the AND operator

function countBitsToFlipAND(a, b) {
    let flips = 0; // initially flips is equal to 0
    let t1, t2;
    
    // Loop through each bit of A and B
    while (a > 0 || b > 0) {
        t1 = a & 1; // Extract the least significant bit of A
        t2 = b & 1; // Extract the least significant bit of B
        
        // If the bits are different, increment the flips count
        if (t1 !== t2) {
            flips++;
        }
        
        // Right shift A and B to check the next bit
        a >>= 1;
        b >>= 1;
    }
    
    // Return the total flips count
    return flips;
}

const A1 = 10, B1 = 20;
console.log("Using AND Operator:", countBitsToFlipAND(A1, B1)); 

Using AND Operator: 4

Time Complexity: O(log(max(A, B)))

Auxiliary Space: O(1)