Converting 1 array of objects into a nested array of array of objects

Question

Say I have this:

var arr = [
    {type:"orange", title:"First"},
    {type:"orange", title:"Second"},
    {type:"banana", title:"Third"},
    {type:"banana", title:"Fourth"}
];

And I'd like to get to this.

[[{type:"orange", title:"First"},
{type:"orange", title:"Second"}],

[{type:"banana", title:"Third"},
{type:"banana", title:"Fourth"}]]

I've seen a lot for getting to this:

{orange: 
[[{type:"orange", title:"First"},
{type:"orange", title:"Second"}], 
banana: 
[{type:"banana", title:"Third"},
{type:"banana", title:"Fourth"}]}

But it doesn't help my situation, I need it to be arrays inside an array. Any help will be greatly appreciated

We need to know how these arrays are getting built. Theres not enough info here to respond to. — tintyethan, Apr 06 '22 at 19:59
data is being imported in from a SQL server and the process cannot be changed on that end — profiter, Apr 06 '22 at 20:04

score 2 · Accepted Answer · answered Apr 06 '22 at 20:03

With one more step, you can map it to the format you want.

var arr = [
    {type:"orange", title:"First"},
    {type:"orange", title:"Second"},
    {type:"banana", title:"Third"},
    {type:"banana", title:"Fourth"}
];


const typeDictionary = {}

for (item of arr) {
  if (!typeDictionary[item.type]) {
    typeDictionary[item.type] = [item]
  } else {
    typeDictionary[item.type].push(item)
  }
}

// This is the final missing step
const finalResult = Object.values(typeDictionary)

console.log('finalResult', finalResult)

finalResult [
  [
    { type: 'orange', title: 'First' },
    { type: 'orange', title: 'Second' }
  ],
  [
    { type: 'banana', title: 'Third' },
    { type: 'banana', title: 'Fourth' }
  ]
]

PeterKA · Answer 2 · 2022-04-06T20:59:59.153

You could use this strategy

Find all the unique types, ['orange','banana']
For each unique type Array#filter the original array
DONE

const input = [
    {type:"orange", title:"First"},
    {type:"orange", title:"Second"},
    {type:"banana", title:"Third"},
    {type:"banana", title:"Fourth"}
];

const output = [...new Set(input.map(o => o.type))]
    .map( type => input.filter(o => o.type === type) )

console.log( output );

Alternatively, you could use Array#reduce as follows:

const input = [
    {type:"orange", title:"First"},
    {type:"orange", title:"Second"},
    {type:"banana", title:"Third"},
    {type:"banana", title:"Fourth"}
];

const output = input.reduce((prev, {type,title}) => {
    let x = prev.findIndex(ar => ar[0].type === type);
    if( x > -1 ) {
        prev[x].push({type,title});
    } else {
        prev.push([{type,title}])
    }
    return prev;
}, []);
    

console.log( output );

This is far less efficient than a standard group-by (O(n*m) vs O(n)), and also this question is a duplicate multiple times over. — pilchard, Apr 06 '22 at 20:46

Converting 1 array of objects into a nested array of array of objects

2 Answers2