Nested For Loops JavaScript

Question

I have an app which gets data from the database (MongoDB) in JSON format. The returned data contains nested arrays and I want to access the values in the nested arrays. The returned JSON format looks like this:

[
 {"_id": "81724587125","name": "Object 1", "arrayOne":["1","2","3"]},
 {"_id": "87687687687","name": "Object 2", "arrayOne":["4","5","6"]}
]

Now what I want is to retrieve the values from arrayOne from all objects and push them into a single array like so:

combinedValues:["1","2","3","4","5","6"]

To achieve this I have used Nested For Loops below is my actual code:

 function myFunc(result){

    $scope.clients = [];

    for(var i = 0; i < result.length; i++){
      for(var x = 0; x < result[i].sectorClients.length; x++){
        $scope.clients.push(result[i].sectorClients[x]);
      }
    }       
};

The above code works fine and does what I want it to do.

My Question:

Is using nested for loops for this particular situation a good programming practice? Is there a better way of achieving the same functionality? Ive heard people say that nested for loops can be "expensive" processing wise.

Answer 1

As I mentioned in the comments you can get the same result directly from MongoDB using a MapReduce query; However a cleaner JavaScript equivalent for your nested loops can be something like this:

var data = [
 {"_id": "81724587125","name": "Object 1", "arrayOne":["1","2","3"]},
 {"_id": "87687687687","name": "Object 2", "arrayOne":["4","5","6"]}
];

var result = data.reduce(function (previousValue, currentValue, currentIndex, array) {
    return previousValue.arrayOne.concat( currentValue.arrayOne );
});

Answer 2

Nested loops are expensive when they both iterate over independent data in a "criss-cross" fashion, because they may represent a O(n^2) algorithm that can be made more efficient. You may think of such loops as describing the program "for every element in X, loop over every element in Y again".

This is not one of those cases. Your data are an array of objects, each containing further arrays of objects. A nested loop here is not only fine: it's completely expressive and appropriate.

Answer 3

in the case, you needn't use "puth" function, push every tiny element into the array;because you can operate the whole array eg: [1,2,3].concat([4,5,6]) => [1, 2, 3, 4, 5, 6]

then you can improve your code by reducing the loop

Answer 4

You can use map function of array to get the properties (arrayOne) you want, and create a flattened array from array of arrays (array of arrayOnes) using reduce.

var clients = results
  .map(function(r){ return r.arrayOne; })
  .reduce(function(prev, curr) { return prev.concat(curr); }, []);
// ["1", "2", "3", "4", "5", "6"]

Resources:

• MDN Array Map
• MDN Array Reduce