Javascript:Find first occurance of 1 in string of zeros followed by 1

Find first occurance of 1 in string of zeros followed by 1s.

Code:

    let s="00000001"

    let left =0;

    let right = s.length-1;

    let ans = -1;

    while(left<=right){

        let mid = Math.floor((left+right)/2);

        if (s[mid]==="1"){

            right = mid-1;

            ans = mid;//it change value of answer progressively

        }else{

            left = mid+1;

        }

    }

    console.log('Ans =',ans)

Output:

    Ans = 7

Explanation:

Purpose: find the index of the first 1 in a string of zeros then ones.

How: binary search — check middle; if it's 1, save index and search left; if 0, search right.

Result: for s = "00000001" prints Ans = 7; returns -1 when no 1 is found.