app.js

const data = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26];
let variants = [];
const f = function (res, i) {
    if (i > data.length - 1){
        variants.push(res);
        return;
    }
    let res1 = res.slice();
    f(res1, i+1);
    let res2 = res.slice()
    res2[i] = data[i];
    f(res2, i+1);
}
f([], 0);
console.log(variants.length);