powerset.js

function powerset(arr) {
    let sets = [[]];
    arr.forEach(value => {
       sets.forEach(set => {
           sets.push(set.concat(value));
       });
    });

    return sets;
}

console.log(powerset(['a', 'b', 'c']));

//output => [ [], [ 'a' ], [ 'b' ], [ 'a', 'b' ], [ 'c' ], [ 'a', 'c' ], [ 'b', 'c' ], [ 'a', 'b', 'c' ] ]