Get keys of first object in object of arrays

Question

I am trying to get an array of (distinct) keys from an array of objects.

arr =[
   {id: 1, desc: "", name: "", objectives: Array(3), …},
   {id: 2, desc: "", name: "", objectives: Array(3), …},
   {id: 3, desc: "", name: "", objectives: Array(3), …},
   {id: 4, desc: "", name: "", objectives: Array(3), …},
]

Desired output is [id, desc, name, objectives]. I have tried:

Object.keys(arr)[0]
// output [{id: 1, desc: "", name: "", objectives: Array(3), …}]

arr.flatMap(Object.key)
// output ["id", "desc", "name", "objectives", "id", "desc", "name", "objectives", "id", "desc", "name", "objectives", "id", "desc", "name", "objectives"]

I'm sorry if I missed the answer to this when scouring SO. Any help would be really appreciated.

The keys of just the first object (as per the title) would just be const keys = Object.keys(arr[0]);. Was that what you're asking? — Robby Cornelissen
– Robby Cornelissen, Commented Oct 10, 2019 at 0:21

Robby Cornelissen · Accepted Answer · 2019-10-10 00:16:12Z

2

You can use a Set to weed out the duplicates:

const arr = [
   {id: 1, desc: "", name: "", objectives: []},
   {id: 2, desc: "", name: "", objectives: []},
   {id: 3, desc: "", name: "", objectives: []},
   {id: 4, desc: "", name: "", objectives: []},
];

const keys = [...new Set(arr.flatMap(Object.keys))];

console.log(keys);

answered Oct 10, 2019 at 0:16

Robby Cornelissen

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1 Comment

Simon Dehaut Over a year ago

sry i delete my comment, i wanted to say he should write Object.keys(arr[0]) instead of Object.keys(arr)[0] (so we agree :) )

pjconcepcion · Accepted Answer · 2019-10-10 01:20:57Z

1

This should work.

arr = [
   {id: 1, desc: "", name: "", objectives: Array(3)},
   {id: 2, desc: "", name: "", objectives: Array(3)},
   {id: 3, desc: "", name: "", objectives: Array(3)},
   {id: 4, desc: "", name: "", objectives: Array(3)},
];

keyArr = [];
Object.keys(arr).find(keys => {
    Object.keys(arr[keys]).find(key => {
    keyArr.push(key);
  });
})
console.log(keyArr);

answered Oct 10, 2019 at 1:20

pjconcepcion

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Comments

EthanLWillis · Accepted Answer · 2019-10-10 00:46:23Z

    const arr =[
      {id: 1, desc: "", name: "", objectives: Array(3), …},
      {id: 2, desc: "", name: "", objectives: Array(3), …},
      {id: 3, desc: "", name: "", objectives: Array(3), …},
      {id: 4, desc: "", name: "", objectives: Array(3), …},
    ];

    let distinct = function(arr, key_name) {
       let seen_values = [];
       let unique_objects = [];
       for(i = 0; i < arr.length; i++) {
         // If this value is new, a.k.a. distinct then add it to our filtered list.
         if(!seen_values.includes(arr[i][key_name]) {
            unique_objects.push(arr[i])
         }
         // Add this value to our seen values.
         seen_values.push(arr[i][key_name])
       }
       return unique_objects;
    }

    // Example usage, filter by desc
    let filtered_set = distinct(arr, "desc")

    // Example usage, filter by id
    let filtered_set_2 = distinct(arr, "id")

So this should work. The distinct function will filter the passed in arr by the given key_name. One limitation of this approach is that if one of the values in your object is yet another object you will need to do deeper inspection of that object to be able to compare them.

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