Generate all possible permutations of a string

permutation.js

function permutation(str, cache) {
    var chars = str.split("");
    if (chars.length == 1)  {
        return chars;
    }
    cache = cache || {};
    if(cache[str]) {
        return cache[str];
    }
    result = chars.reduce(function(res, curr, idx, array){
        var subStr = str.slice(0,idx)+str.slice(idx+1);
        var subResult = permutation(subStr, cache);
        subResult.forEach(function(v){
            res.push(curr+v);
        });
        return res;
    }, []);
    cache[str] = result;
    return result;
}
permutation("abcdefg");