If you want to map objects to something (in this case a property). I think Array.prototype.map is what you're looking for if you want to code functionally.

(fiddle)

If you want to support older browsers, that are not ES5 compliant you can shim it (there is a polyfill on the MDN page above). Another alternative would be to use underscorejs's pluck method:

var users = [
      {name: "Joe", age: 22},
      {name: "Kevin", age: 24},
      {name: "Peter", age: 21}
    ];
var result = _.pluck(users,'name').join(",")

Before join you need map array

const array = [
    { a: '1', b: '2' },
    { a: '3', b: '4' },
];

const result = array.map(_ => _.a).join(', ');

console.log(result);

Are you looking for something like this? You'll need to add types, but you can merge the features arrays from all entries in the data array using vanilla JS methods.

1. Create the output object and specify its type.
2. Use Array#filter to select the entries in data with the matching type.
3. Use Array#flatMap to select the features array from each of the entries above and project their contents into a single array.

const data = [
  { type: 'FeatureCollection', features: [1, 2, 3] },
  { type: 'FeatureCollection', features: [4, 5, 6] }
];

const output = { 
  type: 'FeatureCollection',
  features: data
    .filter(obj => obj.type === 'FeatureCollection')
    .flatMap(obj => obj.features) 
};
  
console.dir(output);

The spread operator is kinda cool.

this.results = [ ...this.results, ...data.results];

The spread operator allows you to easily place an expanded version of an array into another array.

You can read about spread operator here.