Get new array from array of objects [duplicate]

Question

I have an array that has another array inside.

[
    [
        {
          "userId": 1,
          "title": "title 1",
        },
        {
          "userId": 2,
          "title": "title 2",
        }
    ],
    [
        {
          "userId": 3,
          "title": "title 3",
        }
    ]
]

I am trying to get a new array with userId only. For e.g.

[
  { "userId": 1 },
  { "userId": 2 },
  { "userId": 3 }
]

array.map(o => o.userId) works for array of objects and don't know how can i get inside array.

Any help is appreciated

Possible duplicate of JavaScript flattening an array of arrays of objects and Merge/flatten an array of arrays in JavaScript? and How to flatten nested array in javascript? — adiga
– adiga, Commented Feb 4, 2019 at 14:10

Taki · Accepted Answer · 2019-02-04 13:58:58Z

2

you'll have to flat the array first :

const data = [
    [
        {
          "userId": 1,
          "title": "title 1",
        },
        {
          "userId": 2,
          "title": "title 2",
        }
    ],
    [
        {
          "userId": 3,
          "title": "title 3",
        }
    ]
]

const result = data.flat().map(({userId}) => ({userId}));
console.log(result);

answered Feb 4, 2019 at 13:58

Taki

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2 Comments

user5283119 Over a year ago

Literally half way through typing this same answer. This is the easiest way to achieve your outcome.

Praveen Kumar Purushothaman Over a year ago

Same answer. Ahhhh!!! 😭

Hassan Imam · Accepted Answer · 2019-02-04 14:10:46Z

2

You can flatten the array using array#concat and then using destructuring and array#map generate the array.

const data = [ [ { "userId": 1, "title": "title 1", }, { "userId": 2, "title": "title 2", } ], [ { "userId": 3, "title": "title 3", } ] ],
      result = [].concat(...data).map(({userId}) => ({userId}));
console.log(result);

.as-console-wrapper { max-height: 100% !important; top: 0; }

answered Feb 4, 2019 at 14:10

Hassan Imam

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Comments

chazsolo · Accepted Answer · 2019-02-04 14:06:45Z

1

Array.prototype.flat is fairly new; in case you can't use it you can use a combination of reduce and map:

const data = [
    [
        {
          "userId": 1,
          "title": "title 1",
        },
        {
          "userId": 2,
          "title": "title 2",
        }
    ],
    [
        {
          "userId": 3,
          "title": "title 3",
        }
    ]
]

const userIds = data.reduce((_, a) => {
	return _.concat(a.map(({ userId }) => ({ userId })))
}, [])

console.log(userIds)

A benefit about map within the reduce call is that you're only iterating over the array once instead of chaining. This will have better performance over larger arrays than chaining array methods.

^{All assuming your data structure is only one level deep!}

answered Feb 4, 2019 at 14:06

chazsolo

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4 Comments

nicholaswmin Over a year ago

You're iterating once for the enclosing array and then iterating each element with .map . If anything I expect that this would be marginally slower because it needs to create a function for each inner element iteration.

chazsolo Over a year ago

@NikKyriakides not that jsperf is the end-all for the subject, but I decided to run this through and check flat vs this method.. At least in this case, reduce is much faster.

nicholaswmin Over a year ago

Really? Interesting. Can't check your benchmark now but if you say so.

chazsolo Over a year ago

Honestly it surprised me, too!

nicholaswmin · Accepted Answer · 2019-02-04 14:07:28Z

1

Another one using Array.reduce, for browsers that don't support Array.flat.

const data = [
  [
    {
      "userId": 1,
      "title": "title 1",
    },
    {
      "userId": 2,
      "title": "title 2",
    }
  ],
  [
    {
      "userId": 3,
      "title": "title 3",
    }
  ]
]

const result = data.reduce((arr, i) => {
  return arr.concat(i.map(({ userId }) => ({ userId })))
}, [])

console.log(result)

answered Feb 4, 2019 at 14:07

nicholaswmin

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Comments

Silvio Biasiol · Accepted Answer · 2019-02-04 14:01:09Z

-1

Just get everything out in a new array :)

let arr = [
    [
        {
          "userId": 1,
          "title": "title 1",
        },
        {
          "userId": 2,
          "title": "title 2",
        }
    ],
    [
        {
          "userId": 3,
          "title": "title 3",
        }
    ]
]

let newArr = []
arr.forEach(i => i.forEach(o => newArr.push(o)))
console.log(newArr.map(o => o.userId))

answered Feb 4, 2019 at 14:01

Silvio Biasiol

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