const arr = [{
            id: 1,
            fullnames: 'JosephSam',
            weight: 1231,
            currentdate: '2021-09-22'
}];

const data = arr[0];

console.log(data.fullnames); // 'JosephSam'

This would be what you need. I do get the feeling this is isn't what you're looking for. If so, please give us a more complex example to see what it is you're dealing with.

If you have multiple values inside your array, then you can have a separate object to insert each array value into it by having id as the object key.

Take a look at the following example.

const data = [
  {
    id: 1,
    fullnames: "JosephSam",
    weight: 1231,
    currentdate: "2021-09-22",
  },
  {
    id: 2,
    fullnames: "Jane",
    weight: 1431,
    currentdate: "2021-09-22",
  },
];

const mappedData = {};

data.forEach((element) => (mappedData[element.id] = element));

console.log("Mapped Data: ", mappedData);

Console log

Mapped Data:  {
  '1': {
    id: 1,
    fullnames: 'JosephSam',
    weight: 1231,
    currentdate: '2021-09-22'
  },
  '2': { id: 2, fullnames: 'Jane', weight: 1431, currentdate: '2021-09-22' }
}

Then you can refer each element by id as follows. (x is the id you need to get)

mappedData[x]

I'm not sure what are you trying to do. But If you have a single indexed array you should to this:

var data = [{
    id: 1,
    fullnames: 'JosephSam',
    weight: 1231,
    currentdate: '2021-09-22'
}]
data = data[0];