Return array of objects (from array of objects)

Question

I have the following data and I want to return an array (of objects) of years that are distinct.

I tried the following function but I'm getting an array within an array.

const data = [{
    id: 1,
    name: "test1",
    years: [{
      id: 1,
      name: "year1"
    }, {
      id: 2,
      name: "year2"
    }]
  },
  {
    id: 2,
    name: "test2",
    years: [{
      id: 1,
      name: "year1"
    }]
  },
]

let years = data.map((s) => {
  return s.years
})
let distinctYears = Array.from(new Set(years.map(c => c.id))).map(id => {
  return {
    id: id,
    name: years.find(c => c.id === id).name,
  }
})

console.log(distinctYears);

desired outcome:

[
  {id: 1, name: "year1"},
  {id: 2, name: "year2"}
]

I want to return both, but distinct based on id. I included the desired outcome in my question. — jj008
– jj008, Commented Aug 20, 2019 at 18:37

Barmar · Accepted Answer · 2019-08-20 18:41:18Z

2

Since s.years() is an array, and data.map() returns an array of the results, years is necessarily an array of arrays.

Instead of using .map(), use .reduce() to concatenate them.

const data = [{
    id: 1,
    name: "test1",
    years: [{
      id: 1,
      name: "year1"
    }, {
      id: 2,
      name: "year2"
    }]
  },
  {
    id: 2,
    name: "test2",
    years: [{
      id: 1,
      name: "year1"
    }]
  },
];

const years = data.reduce((a, {
  years
}) => a.concat(years), []);
let distinctYears = Array.from(new Set(years.map(c => c.id))).map(id => {
  return {
    id: id,
    name: years.find(c => c.id === id).name,
  }
});
console.log(distinctYears);

answered Aug 20, 2019 at 18:41

Barmar

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Comments

Cat_Enthusiast · Accepted Answer · 2019-08-20 18:48:05Z

There's so many ways you can go about doing this. Here's one, it's not a one-liner but its broken down to parts to help us understand whats going on.

Your dataset:

let data =
[
  {
    id: 1,
    name: "test1",
    years: [{id: 1, name: "year1"}, {id: 2, name: "year2"} ]
  },
  {
    id: 2,
    name: "test2",
    years: [{id: 1, name: "year1"} ]
  },
]

Use .flatMap() to create a one-level array with all items:

let allItems = data.flatMap((item) => {
    return item.years.map((year) => {
        return year
    })
})

Getting distinct items:

let distinct = []

allItems.forEach((item) => {
    let matchingItem = distinct.find((match) => match.id == item.id && match.name == item.name)

    if(!matchingItem){
        distinct.push(item)
    }
})

In Practice:

let data = [{
    id: 1,
    name: "test1",
    years: [{
      id: 1,
      name: "year1"
    }, {
      id: 2,
      name: "year2"
    }]
  },
  {
    id: 2,
    name: "test2",
    years: [{
      id: 1,
      name: "year1"
    }]
  },
]

let allItems = data.flatMap((item) => {
  return item.years.map((year) => {
    return year
  })
})

let distinct = []

allItems.forEach((item) => {
  let matchingItem = distinct.find((match) => match.id == item.id && match.name == item.name)

  if (!matchingItem) {
    distinct.push(item)
  }
})

console.log(distinct)

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