Getting a substructure in a JSON-file
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 =&gt; father.daughters);