Javascript：从对象数组创建二维数组

Question

I have the following array of objects and want to have a 2d array from it. What I have tried so far:

let arr = [
  {
    "id": '1',
    "userId": '1',
    "postId": 1,
    "type": 'like'
  },
  {
    "id": '2',
    "userId": '1',
    "postId": 2,
    "type": 'like'
  },
  {
    "id": '3',
    "userId": '2',
    "postId": 3,
    "type": 'like'
  },
  {
    "id": '4',
    "userId": '2',
    "postId": 4,
    "type": 'dislike'
  },
  {
    "id": '5',
    "userId": '2',
    "postId": 1,
    "type": 'disklike'
  },
];

var newArr = arr.map(function(val, index){          
 return [val.type]
})

This returns:

[
    ["like"], ["like"], ["like"], ["dislike"], ["dislike"]
]

Which is not the expected results, The expected results are:

[
    ["like", "like"], // User 1
    ["like", "dislike", "dislike"] // User 2
]

as user with userId 1 have two likes and user with userId 2 have one like and two dislikes.

What am I missing here.

Answer 1

You can use:

 const original = [ { id: '1', userId: '1', postId: 1, type: 'like' }, { id: '2', userId: '1', postId: 1, type: 'like' }, { id: '3', userId: '2', postId: 3, type: 'like' } ]; const newArray = original.reduce ((array, item) => { const index = parseInt(item.userId) - 1; array[index] = array[index]?? []; array[index].push(item.type); return array; }, []); console.log(newArray);