combinations.js

const getCombinations = (map, digitCount) => {
    let numCombinations = Math.pow(map.length, digitCount);
    const retv = [];
    for(let i = 0; i < numCombinations; i++) {
        let current = [];
        let val = i;
        do {
            let idx = Math.floor(val % map.length);
            current.push(map[idx]);
            val = Math.floor(val / map.length);
        } while ((val) > 0);
        if(current.length < digitCount) {
            current = current.concat(new Array(digitCount - current.length).fill(map[0]));
        }
        current = current.reverse();
        retv.push(current.join(''));
    }
    return retv;
}

// use: getCombinations('1234', 4);