[解決済み] null 値が常に最後に来るように配列を並べ替える

function alphabetically(ascending) {

  return function (a, b) {

    // equal items sort equally
    if (a === b) {
        return 0;
    }
    // nulls sort after anything else
    else if (a === null) {
        return 1;
    }
    else if (b === null) {
        return -1;
    }
    // otherwise, if we're ascending, lowest sorts first
    else if (ascending) {
        return a < b ? -1 : 1;
    }
    // if descending, highest sorts first
    else { 
        return a < b ? 1 : -1;
    }

  };

}

var arr = [null, 'a', 'b', null, 'd'];

console.log(arr.sort(alphabetically(true)));
console.log(arr.sort(alphabetically(false)));