Using reduce, from an array of objects, create a set of elements inside the objects' array attributes

Question

Given the following array:

foos = [
  {
    id: 0,
    bar: ['a','b','c']
  },
  {
    id: 1,
    bar: ['a','b','d']
  },
  {
    id: 2,
    bar: ['a','c']
  },
]

Using reduce, how can I achieve the following?:

bars == ['a','b','c','d']

I've tried:

foo.reduce((bars, foo) => bars.add(foo.bar), new Set())

But it results in a set of objects:

Set { {0: 'a', 1: 'b', 2: 'c'}, {0: 'a', 1: 'b', 2: 'd'}{0: 'a', 1: 'c'}}

And:

foos.reduce((bars, foo) => foo.bar.forEach(bar => bars.add(bar)), new Set())

But the forEach has no access to the bars set.

Use map inside reduce, you are pushing bar array into set, that is pushing whole bar object into set — Awais
– Awais, Commented Oct 20, 2021 at 12:07
You need to loop through foo.bar and add them individually to bars.add(). And return the bars set from the reduce — adiga
– adiga, Commented Oct 20, 2021 at 12:12

StackByMe · Accepted Answer · 2021-10-20 12:13:34Z

5

Instead of creating a Set inside your reduce. You could just reduce all bar arrays into a single one and pass that to your Set constructor.

const foos = [
  {
    id: 0,
    bar: ['a','b','c']
  },
  {
    id: 1,
    bar: ['a','b','d']
  },
  {
    id: 2,
    bar: ['a','c']
  },
];

const bars = new Set(foos.reduce((all, foo) => [...all, ...foo.bar], []));

console.log(...bars);

With flatMap:

const foos = [
  {
    id: 0,
    bar: ['a','b','c']
  },
  {
    id: 1,
    bar: ['a','b','d']
  },
  {
    id: 2,
    bar: ['a','c']
  },
];

const bars = new Set(foos.flatMap(foo => foo.bar));

console.log(...bars);

answered Oct 20, 2021 at 12:13

StackByMe

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Comments

darklightcode · Accepted Answer · 2021-10-20 12:30:34Z

3

You can concat the bar property in the accumulator, and use .filter method to make the values unique:

const foos = [{
    id: 0,
    bar: ['a', 'b', 'c']
  },
  {
    id: 1,
    bar: ['a', 'b', 'd']
  },
  {
    id: 2,
    bar: ['a', 'c']
  },
];

const bars = foos
  .reduce((acc, itm) => acc.concat(itm.bar), [])
  .filter((i, x, s) => s.indexOf(i) === x);

console.log(...bars);

answered Oct 20, 2021 at 12:30

darklightcode

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