How to group by the same key in a collection

Question

Assume the following objects

let oldArr = [
  { a: 1 },
  { a: 2 },
  { a: 3 },
  { b: 1 },
  { b: 2 },
  { c: 1 }
]

Desired result

let newArr = [
  [
    { a: 1 },
    { a: 2 },
    { a: 3 },
  ],
  [
    { b: 1 },
    { b: 2 },
  ],
  [
    { c: 1 }
  ],
]

I try to use lodash, I see the partition function but it only splits the arrays into 2 groups. The groupBy groups it into an object by keys. Is there any good way? hope to get everyone's help, thank you!

Answer 1

Simply like this:

let oldArr = [
  { a: 1 },
  { a: 2 },
  { a: 3 },
  { b: 1 },
  { b: 2 },
  { c: 1 }
]

let objects = {};

for (let item of oldArr) {
  // get key
  let key = Object.keys(item)[0];
  // check if this key added before
  if (!objects[key]) objects[key] = [];
  // push this object
  objects[key].push(item);
}

let newArr = Object.values(objects);

console.log(newArr)

But you should be sure that every object in oldArr has only one key.

Answer 2

you can do this using Object.keys, Object.values and Array.reduce

let oldArr = [
  { a: 1 },
  { a: 2 },
  { a: 3 },
  { b: 1 },
  { b: 2 },
  { c: 1 },
  { a: 4, c: 2 }
]

let newArr = 
  // drop the keys of the object created by the reduce
  Object.values(
    // for each object in the array
    oldArr.reduce((acc, el) => {
      // for each keys in the object
      Object.keys(el).forEach(key => {
        // add the object to the group of objects with this key
        acc[key] = acc[key] || []
        acc[key].push(el)
      })
      return acc
    }, {})
  )

console.log(newArr)

if an object in the input have multiple keys it will go to each group in the output

Answer 3

you can use like that

let oldArr = [
  { a: 1 },
  { a: 2 },
  { a: 3 },
  { b: 1 },
  { b: 2 },
  { c: 1 }
];
let newArr = {};
oldArr.forEach((i)=>{
  let key = Object.keys(i)[0];
  newArr[key] = newArr[key] || [];
  newArr[key].push(i);
});
newArr = Object.values(newArr);
console.log(newArr);