Write a pure function to return one object from inner properties another objects?

Question

Doing some data transformation exercises and getting stuck. I have an object that I want to transform to look likefrom (starting) -> to (expected ending) output described below. I'm trying to use Array.reduce and Object.assign to keep the output pure. but I just cannot get it to work properly.

/**
 *  from (starting): {topic: {id: 2}, products: {id: 3}}
 *  to (expected ending):   {topic: 2, products: 3}
 */

const starting = {topic: {id: 2}, products: {id: 3}};

const ending = Object.keys(starting).reduce((p, key) => {
  if(!p[key]) p[key] = key;
  return Object.assign(p[key], starting[key].id);  
}, {})

Answer 1

You can use reduce, just remember to return the starting object after each iteration.

function convert(start) {
  return Object.keys(starting).reduce((o, key) => {
      o[key] = start[key].id;
      return o;
    }, {});
}

const starting = {topic: {id: 2}, products: {id: 3}};
console.log(convert(starting));

Answer 2

Try this:

var starting = {topic: {id: 2}, products: {id: 3}};

var ending = Object.keys(starting).reduce((p, key) => {
  p[key] = starting[key].id;
  return p;
}, {})

This will create a new object, so no need for Object.assign and such to keep the output pure.

Answer 3

I don't think reduce() is the right tool for this.

Try iterating through starting's keys using forEach() instead:

const starting = {topic: {id: 2}, products: {id: 3}};

const ending = {};
Object.keys(starting).forEach(p => ending[p] = starting[p].id);

console.log(ending);

Answer 4

A pure function solution would require you to return a new object on each iteration:

function convert(start) {
  return Object.keys(starting).reduce((o, key) => 
    Object.assign({}, {
      [key]: start[key].id
    }, o), {});
}

const starting = {topic: {id: 2}, products: {id: 3}};

console.log(convert(starting));

Using object spread makes it a bit cleaner:

function convert(start) {
  return Object.keys(starting).reduce((o, key) => ({
    ...o,
    [key]: start[key].id
  }), {});
}

const starting = {topic: {id: 2}, products: {id: 3}};

console.log(convert(starting));