How can I reverse an array of object in Javascript?

Question

I have an array written in this way:

[
  { TestMetaCoin: 'L', 'Contract: MetaCoin': 'K' },
  { TestMetaCoin: 'L', 'Contract: MetaCoin': 'L' },
  { TestMetaCoin: 'L', 'Contract: MetaCoin': 'K' },
  { TestMetaCoin: 'L', 'Contract: MetaCoin': 'L' }
]

And I want to obtain an array writtenin this way:

[
  { L: 'TestMetaCoin', K: 'Contract: MetaCoin' },
  { L: ['TestMetaCoin','Contract: MetaCoin'] },
  { L: 'TestMetaCoin', K: 'Contract: MetaCoin'  },
  { L: ['TestMetaCoin','Contract: MetaCoin'] }
]

I tryed to use the nodejs function restore() but the result is:

[
  { L: 'TestMetaCoin', K: 'Contract: MetaCoin' },
  { L: 'Contract: MetaCoin' },
  { L: 'TestMetaCoin', K: 'Contract: MetaCoin' },
  { L: 'Contract: MetaCoin' }
]

It completely overwrites the value in position 2 and 4, probably because it tries to create an existing key and therefore not being able to create it, accesses the old one by overwriting the value exists instead of creating an array with 2 values. How can I get around this problem? This is the code that I use:

var fristArray= [
  { TestMetaCoin: 'L', 'Contract: MetaCoin': 'K' },
  { TestMetaCoin: 'L', 'Contract: MetaCoin': 'L' },
  { TestMetaCoin: 'L', 'Contract: MetaCoin': 'K' },
  { TestMetaCoin: 'L', 'Contract: MetaCoin': 'L' }
]
var swapped=[]
for(var i=0;i<fristArray.length;i++) {
swapped.push(Object.fromEntries(Object.entries(fristArray[i]).map(([k, v]) => [v, k])))
}
console.log(swapped)

trincot · Accepted Answer · 2022-02-17 15:16:16Z

You can first create the target object with the new keys and for each an empty array as value. Then populate those arrays. Finally replace any of those arrays by a single value when they happen to only have one element. Else leave them as arrays:

function swapper(obj) {
    let result = Object.fromEntries(Object.values(obj).map(value => [value, []]));
    for (let [key, value] of Object.entries(obj)) result[value].push(key);
    for (let [key, value] of Object.entries(result)) {
        if (value.length === 1) result[key] = value[0];
    }
    return result;
}

var firstArray = [{ TestMetaCoin: 'L', 'Contract: MetaCoin': 'K' },{ TestMetaCoin: 'L', 'Contract: MetaCoin': 'L' },{ TestMetaCoin: 'L', 'Contract: MetaCoin': 'K' },{ TestMetaCoin: 'L', 'Contract: MetaCoin': 'L' }];
var swapped = firstArray.map(swapper);
console.log(swapped);

Alternative

Here we store a single value first, and then turn it into an array when needed.

function swapper(obj) {
    let result = {};
    for (let [key, value] of Object.entries(obj)) {
        result[value] = value in result ? [key].concat(result[value]) : key;
    }
    return result;
}

var firstArray = [{ TestMetaCoin: 'L', 'Contract: MetaCoin': 'K' },{ TestMetaCoin: 'L', 'Contract: MetaCoin': 'L' },{ TestMetaCoin: 'L', 'Contract: MetaCoin': 'K' },{ TestMetaCoin: 'L', 'Contract: MetaCoin': 'L' }];
var swapped = firstArray.map(swapper);
console.log(swapped);

Hao Wu · Accepted Answer · 2022-02-17 15:16:29Z

2

Here's an elaborated one-liner solution using nothing but ES6 functions if you're interested.

var fristArray = [
  { TestMetaCoin: 'L', 'Contract: MetaCoin': 'K' },
  { TestMetaCoin: 'L', 'Contract: MetaCoin': 'L' },
  { TestMetaCoin: 'L', 'Contract: MetaCoin': 'K' },
  { TestMetaCoin: 'L', 'Contract: MetaCoin': 'L' }
]

var swapped = fristArray.map(obj => 
  Object.entries(obj).reduce((acc, [k, v]) => ({ ...acc, [v]: acc[v] ? [acc[v], k].flat() : k }), {})
);

console.log(swapped);

answered Feb 17, 2022 at 15:16

Hao Wu

21.6k7 gold badges37 silver badges79 bronze badges

2 Comments

trincot Over a year ago

reduce and map are ES5, right? (+1)

Hao Wu Over a year ago

@trincot You're right, sorry for being not precise enough but you get the idea :P

Eugenio · Accepted Answer · 2022-02-17 15:16:23Z

1

You can try something like this:

const invertKeyValues = (obj, fn) =>
  Object.keys(obj).reduce((acc, key) => {
    const val = fn ? fn(obj[key]) : obj[key];
    acc[val] = acc[val] || [];
    acc[val].push(key);
    return acc;
  }, {});

answered Feb 17, 2022 at 15:16

Eugenio

111 bronze badge

Comments

Parvesh Kumar · Accepted Answer · 2022-02-17 15:42:25Z

0

const arr = [
  { TestMetaCoin: 'L', 'Contract: MetaCoin': 'K' },
  { TestMetaCoin: 'L', 'Contract: MetaCoin': 'L' },
  { TestMetaCoin: 'L', 'Contract: MetaCoin': 'K' },
  { TestMetaCoin: 'L', 'Contract: MetaCoin': 'L' },
];
const reverseArr = arr.map(obj => {
  const newObj = {};
  Object.keys(obj).forEach(key => {
    const newKey = obj[key];
    if (newObj[newKey]) {
      if (Array.isArray(newObj[newKey])) {
        newObj[newKey].push(key);
      } else {
        newObj[newKey] = [newObj[newKey], key];
      }
    } else {
      newObj[newKey] = key;
    }
  });
  return newObj;
}
);
console.log(reverseArr);

answered Feb 17, 2022 at 15:42

Parvesh Kumar

1,1447 silver badges16 bronze badges

Collectives™ on Stack Overflow

How can I reverse an array of object in Javascript?

4 Answers 4

Alternative

Comments

2 Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

Alternative

Comments

2 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related