const input = ['a', 'b', 'c'];

const possibilities = Math.pow(2, input.length);

const allsets = [];

for(let i = 0; i < possibilities; i ++) {
  const set = [];
  for( let j =0; j < input.length; j ++) {
    const stuff = Math.pow(2, j);
    if (i & stuff) {
      set.push(input[j]);
    }
  }
  allsets.push(set);
}

console.log(allsets.sort((a, b) => a.length - b.length))