strperm.js

function perm1(input /* string */) {
    if (!input.length) return [input];
    let results = input.split('').reduce((results, c) => {
        let tail = input.split(c).join('');
        return results.concat(perm1(tail).map(p => c + p));
    }, []);
    return results;
}

function perm2(input /* string */) {
    const len = input.length;
    if (!len) return [input];
    let results = [];
    for (let i = 0; i < len; i++) {
        let head = input[i];
        let tail = input.split(head).join('');
        results = results.concat(perm2(tail).map(p => head + p));
    }
    return results;
}

let now = Date.now();
for (let i = 0; i < 100000; i++) perm1('abcd');
let later = Date.now();
console.log((later - now) / 1000, 'seconds');

now = Date.now();
for (let i = 0; i < 100000; i++) perm2('abcd');
later = Date.now();
console.log((later - now) / 1000, 'seconds');