Use map function to create one array from Object that has arrays in it

Question

Using Javascript I am trying to create one array from a JSON object. Problem is is that I am trying to get the acceptedAnswers which is an array. I figured map was the best function for this. However what is returned is an array of arrays. I want to get on array of the values only. Thanks for your help.

Code

let categories = [
      {
        "categoryID": "1",
        "categoryName": "Fruits with seeds",
        "acceptedAnswers": [
          "2","5"
        ]
      },
      {
        "categoryID": "2",
        "categoryName": "Fruits without seeds",
        "acceptedAnswers": [
          "1","3","4"
        ]
      },
      {
        "categoryID": "3",
        "categoryName": "Blue Fruit",
        "acceptedAnswers": [
          "1"
        ]
      }
    ]

JS

let catAcceptedAnswerArray = categories.map(function (cat) {
 return cat.acceptedAnswers
})
console.log(catAcceptedAnswerArray)

Returns:

0: ["2","5"]
1: ["1","3","4"]
2: ["1"]

I am trying to return:

0: ["2","5","1","3","4","1"]

Here is a fiddle

T.J. Crowder · Accepted Answer · 2018-08-06 14:59:07Z

4

map isn't the right tool to reach for here. Simply create an array and append each object's entries to it:

const result = [];
for (const category of categories) {
    result.push(...category.acceptedAnswers);
}

Or if you can't use ES2015+ features, an ES5 version:

var result = [];
categories.forEach(function(category) {
    result.push.apply(result, category.acceptedAnswers);
});

This assumes there won't be thousands and thousands of acceptedAnswers; a single call to push has platform-specific limits (but again, they're in the thousands).

Like almost all array operations, it's possible to shoehorn this into reduce, but you don't gain anything by it:

var result = categories.reduce(function(arr, category) {
    arr.push.apply(arr, category.acceptedAnswers);
    return arr;
}, []);

edited Aug 6, 2018 at 14:59

answered Aug 6, 2018 at 14:47

T.J. Crowder

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2 Comments

hsz Over a year ago

You have my sword.

bilcker Over a year ago

Thanks for that it worked as advertised! I used your first example.

hsz · Accepted Answer · 2018-08-06 14:48:50Z

2

With ES6 you can try with:

categories
  .map(category => category.acceptedAnswers)
  .reduce((acc, ids) => acc.concat(ids), [])

edited Aug 6, 2018 at 14:48

answered Aug 6, 2018 at 14:44

hsz

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2 Comments

T.J. Crowder Over a year ago

It's hideously inefficient, but yes, you can. :-)

T.J. Crowder Over a year ago

Slightly. :-) concat creates new array too. There's no need for any temporary arrays here.

Luca Kiebel · Accepted Answer · 2018-08-06 14:48:06Z

1

You can simply reduce the Array to first level after mapping using Array#concat

let categories = [{
    "categoryID": "1",
    "categoryName": "Fruits with seeds",
    "acceptedAnswers": [
      "2", "5"
    ]
  },
  {
    "categoryID": "2",
    "categoryName": "Fruits without seeds",
    "acceptedAnswers": [
      "1", "3", "4"
    ]
  },
  {
    "categoryID": "3",
    "categoryName": "Blue Fruit",
    "acceptedAnswers": [
      "1"
    ]
  }
]

let catAcceptedAnswerArray = categories.map(cat => cat.acceptedAnswers).reduce((acc, val) => acc.concat(val), []);

console.log(catAcceptedAnswerArray)

answered Aug 6, 2018 at 14:48

Luca Kiebel

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