Given an integer n, return the decimal value of the binary string formed by concatenating the binary representations of 1 to n in order, modulo 109 + 7.

concatenatedBinary.js

/**
 * @param {number} n
 * @return {number}
 */
var concatenatedBinary = function(n) {
    const MOD = 1_000_000_007;
    let ans = 0;
    let bitLength = 0;
    let pow2 = 1; // this will always store 2^bitLength % MOD

    for (let i = 1; i <= n; i++) {
        // If i is a power of two, bitLength increases
        if ((i & (i - 1)) === 0) {
            bitLength++;
            pow2 = (pow2 * 2) % MOD; // update 2^bitLength safely
        }

        // Shift left by multiplying with pow2 (instead of ans << bitLength)
        ans = (ans * pow2 + i) % MOD;
    }

    return ans;
};