function sortByKeys(arr, ...keys) {
  // sort arr by keys
  // keys can be:
  // 1. property name
  // 2. -property name
  // 3. function arr object => value
  let keyToInt = (e1,e2,k, factor) => {
	let v1 = k(e1);
	let v2 = k(e2) ;
	return v1>v2 ? 1*factor : (v1==v2 ? 0 : -1*factor)
    };
    let keysToInt = (e1, e2, keys) => {
        for (let k of keys) {
            let kPred = null
            let factor = 1;
            if (typeof k == 'string') {
                if (k[0] == '-') {
                    kPred = e => e[k.substr(1)];
                    factor = -1;
                } else {
		    kPred = e => e[k];
                }
            } else {
                kPred = k
            }
            let res = keyToInt(e1,e2,kPred, factor);
            if (res !=0) {
                return res;
            }
        }
        return 0;
    }
    return arr.sort((e1,e2) => keysToInt(e1,e2,keys));
}