How to get a particular attribute from an array of array objects?

Question

I have an Array of Arrays, and each Array consists of objects. Here is a simplified version of what I'm referring to (it is a console.log of my original array) -

Array - [Array(2), Array(3), Array(2)]

Each Array has objects in the following format (taking the first array from above) -

Array(2) - 
0: {name: "test", score:40, date: "2018-09-18T00:00:00.000Z"}
1: {name: "test2", score:50 date: "2018-09-18T00:00:00.000Z"}

The other arrays are similar with the same attributes and different values.

I am trying to fetch the name attribute from each of these objects. I tried the below code - but I end up getting an undefined value:

const test1= array1.map(x=> x.values) // this gives me the array of arrays
const test2 = test1.map(function(y){return y.name})// this is my attempt to get the 'name' attribute from all of the arrays that include the objects.

What am I missing out on here? Is there a better way to get the attribute using arrow functions?

Answer 1

Flatten it, and map it to names or Vise versa

First flatten it, and map

const array = [[{name: 'test1'}, {name: 'test2'}], [{name: 'test3'}, {name: 'test4'}]]

var res = [].concat(...array).map(({name})=>name);
console.log(res);

Now, map it to names and then flatten

const array = [[{name: 'test1'}, {name: 'test2'}], [{name: 'test3'}, {name: 'test4'}]]

var res = [].concat(...array.map(a=>a.map(b=>b.name)))
console.log(res);

Now, In this one, certainly you can notice that we are actually mapping it in each level (we have to, no other way with first map only approach. so we can perform a reduce in place of the outer map and concat it there itself, so we can avoid the outer concat (for flatten) and inner concat will actually flatten it. Here we go:

const array = [[{name: 'test1'}, {name: 'test2'}], [{name: 'test3'}, {name: 'test4'}]]

var res = array.reduce((r, a)=>r.concat(a.map(b=>b.name)), []);
console.log(res);

Answer 2

Take into account that map returns an array; you iterate over it. Filter or reduce do the same.

const test1= array1.map(x=> x.values) // x doesn't have a property named  "value"
//simply use forEach
array1.forEach((el) => console.log(el.name))

If you want to capture the names inside a collection:

const let container = [];
array1.forEach((el) => container.push(el.name))

A good way to better understand this iterator functions would be to first use loops and then attempt to "translate" your code into one of them.

Answer 3

Because in your first map x is an array, not an object. So, there is no value. You should map inner arrays then get the desired value.

const arr = [
  [
    {
      name: "test",
      score: 40,
      date: "2018-09-18T00:00:00.000Z"
    },
    { name: "test2", score: 50, date: "2018-09-18T00:00:00.000Z" }
  ],
  [
    {
      name: "foo",
      score: 40,
      date: "2018-09-18T00:00:00.000Z"
    },
    { name: "bar", score: 50, date: "2018-09-18T00:00:00.000Z" }
  ]
];

const test1 = arr
  .map(x => x.map(y => y.name))
  .reduce((acc, el) => [...acc, ...el], []);


console.log(test1);

Answer 4

This should work fine. You need to flatten the array structure and map the names.

const array = [[{name: 'test1'}, {name: 'test2'}], [{name: 'test3'}, {name: 'test4'}]]

const names = array.reduce((acc, innerArray) => {
  return [...acc, ...innerArray.map(entry => entry.name)]
}, [])

console.log(names)

Answer 5

Here:

const arr = [
    [{name: 'a', date:'x'}, {name: 'b', date:'y'}],
    [{name: 'c', date:'x'}, {name: 'd', date:'y'}]
];

const names = arr.map(el => el.map(obj => obj.name));

console.log(names.join());
console.log(names.flat());

you can use flat() to keep names in an array or join() to merge the names into a string.

Answer 6

const test1= array1.map(x=> x.values)

This is returning undefined.

let requiredArr = [];

let array1 = [Array(2), Array(3), Array(2)]

let test2 = array1.map(x => x.map(y => requiredArr(y.name));

test2 will give the desired result.

Answer 7

Adding to Koushik's example, with ES2019, you can use flat() to flatten nested arrays:

const array = [[{name: 'test1'}, {name: 'test2'}], [{name: 'test3'}, {name: 'test4'}]]
var res = array.flat().map( ({name}) => name );
console.log(res);

Or if you have deeper levels:

const array = [[[{name: 'test1'}], {name: 'test2'}], [{name: 'test3'}, {name: 'test4'}]]
var res = array.flat(2).map( ({name}) => name );
console.log(res);

And so on.