function getSubsets(n = '') {
    const array = Array.from(n);
    const base = [''];

    const results = array.reduce((previous, element) => {
        const previousPlusElement = previous.map(el => {
            return `${el}${element}`;
        });
        return previous.concat(previousPlusElement);
    }, base);

    return `Input: ${n.length} --> result:${results} --> output: ${results.length}`;
}

console.log(getSubsets('')); // Input: 0 --> result: --> output: 1
console.log(getSubsets('a')); // Input: 1 --> result:,a --> output: 2
console.log(getSubsets('ab')); // Input: 2 --> result:,a,b,ab --> output: 4
console.log(getSubsets('abc')); // Input: 3 --> result:,a,b,ab,c,ac,bc,abc --> output: 8