two's complement in JS

twos-complement.js

// https://www.cs.cornell.edu/~tomf/notes/cps104/twoscomp.html
// https://www.geeksforgeeks.org/efficient-method-2s-complement-binary-string/
const twosComplement = (digits) => {
    let twosComplementValue = '';
    const lastIndexOfOne = digits.lastIndexOf('1');
    if(lastIndexOfOne === -1) {
        return '1' + digits;
    }

    for(let i=0; i < lastIndexOfOne; i++) {
        twosComplementValue += flipBit(digits[i]);
    }
    twosComplementValue += digits.substring(lastIndexOfOne);
    return twosComplementValue;
}

const flipBit = (bit) => bit === '1' ? '0' : '1';