Q:

How to add to an array within an object

I have this object and I'm trying to convert it.
names = {
  animals: ['Dog', 'Cat', 'Snake'],
  flowers: ['Rose', 'Daisy']
}

I want to convert it into something like this
newObject = {
  type: ['animals', 'flowers']
  subtype: ['Dog', 'Cat', 'Snake', 'Rose', 'Daisy']
}

I wrote the following function but for some reason it's not populating correctly.
const newObject = {};

Object.keys(names).map(type => {
  newObject['types'] = [...newObject['types'], type];
      names[type].map(subtype => {
        newObject['subtypes'] = [...newObject['subtypes'], subtype];
      })
    })

A:

Your .map() callback should return what you want each element to turn into, so the callback behaves somewhat like a transformation function.
However, for a case like this, you don't need to use .map() and can just extract the keys and values from the object. As your values are arrays, you can use .flat() to merge the array of arrays into the one larger array of values for your subtype key:

const names = {
  animals: ['Dog', 'Cat', 'Snake'],
  flowers: ['Rose', 'Daisy']
}

const type = Object.keys(names);
const subtype = Object.values(names).flat();

const newObject = {type, subtype};
console.log(newObject);

