Getting a substructure in a JSON-file

Question

I'm currently playing with D3.js.

While doing so I use this JSON-file as a data source: http://codepen.io/chriscoyier/pen/taqCe

Example of the structure:

{
  "fathers" : [
    {
      "id" : 0,
      "married" : false,
      "name" : "Robert Wilson",
      "sons" : null,
      "daughters" : [
        {
          "age" : 21,
          "name" : "Nancy"
          },
        {
          "age" : 30,
          "name" : "Amy"
          }
        ]
      },
    {
      "id" : 1,
      "married" : true,
      "name" : "Robert Thomas",
      "sons" : null,
      "daughters" : [
        {
          "age" : 22,
          "name" : "Melissa"
          },
        {
          "age" : 6,
          "name" : "Susan"
          }
        ]
      },
      ...
    ]
  }

I'm only interested in "daughters". I have mapped the objects into a flat array structure. The code I used:

var fathers = data.fathers,
      separator = '-------------------',
      persons = []; // collector array

   // Filling the collector array with the 
   //   desired data.
   fathers.forEach(function(value) {

     value['daughters'].forEach(function(value) {
       persons.push(value);
     });

   });

It works fine. My complete code here: http://codepen.io/mizech/pen/WrNzJe

But I wonder: Aren't there better ways (in native JavaScript, no frameworks) then iterating?

Would be pleased if anyone has an idea and would share it.

Sumurai8 · Accepted Answer · 2015-12-05 10:41:26Z

What you are effectivelly doing is reducing your original data-structure to the data you actually need. Effectively, you would not do much different:

var persons = data.fathers.reduce( function( prev, elem ) {
  prev.push.apply( prev, elem.daughters );
  return prev;
}, [] );

You still loop through all values, and "push" all daughters into the Array you are building.

One change I would suggest is to not use the construction

var a = something,
    b = something else,
    c = "yellow";

This is prone to errors. If you forget a comma, or type a semicolon instead of a comma, you drop some of your variables into the global namespace.

Just type "var" for every variable:

var a = something;
var b = something else;
var c = "yellow";

Considering the declarations: You absolutely right. Haven't thought that way. Thanks for the tips. — x32
– x32, Commented Dec 5, 2015 at 10:49

mjolka · Accepted Answer · 2015-12-05 10:39:41Z

The operation you want is called flatMap, SelectMany, or concatMap (I might have missed a few names.)

Map a function over a collection and flatten the result by one-level

As far as I'm aware, this isn't provided out-of-the-box but I found an implementation here:

Array.prototype.flatMap = function(lambda) { 
  return Array.prototype.concat.apply([], this.map(lambda)); 
};

Then you can just write

var persons = data.fathers.flatMap(function(father) { return father.daughters; });

Or if you can use arrow functions,

var persons = data.fathers.flatMap(father => father.daughters);

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