Get all possible combinations with length n of string

combination.js

function combination(str, n) {
    var chars = str.split("");
    n = Math.min(chars.length, n);
    var result = [[]];
    for (var i=0; i<n; i++) {
        var newResult = [];
        result.forEach(function(item) {
            var lastIndex = -1;
            if (item.length>0){
                lastIndex = item[item.length-1];
            }
            var startIndex = lastIndex+1;
            for (var j=startIndex;j<chars.length-n+1+i;j++) {
                newResult.push(item.concat(j))
            }
        });
        result = newResult;
    }
    return result.map(function(item){
        return item.map(function(index){
            return chars[index];
        }).join("");
    });
}

combination("alexislagante",3);
combination("abcdefghijklmnopqrstuvwxyz",5);