Setting property of objects in array

Question

I have an array of objects:

[{
   id: 1,
   cod: '123',
   val: true      
},
{
   id: 2,
   cod: '123',
   val: true      
},
{
   id: 3,
   cod: '123',
   val: true      
},
{
   id: 4,
   cod: '456',
   val: true      
},
{
   id: 5,
   cod: '456',
   val: true      
}]

I need to maintain the val property true for the objects which have the same cod and have the higher id. For example, I'd need the following output:

[{
   id: 1,
   cod: '123',
   val: false
},
{
   id: 2,
   cod: '123',
   val: false
},
{
   id: 3,
   cod: '123',
   val: true      
},
{
   id: 4,
   cod: '456',
   val: false
},
{
   id: 5,
   cod: '456',
   val: true      
}]

How could I change this array in order to obtain the desired results? Should I use filter? I'm a little lost.

"Should I use filter?" Probably not. Is the array always going to be sorted by ID? — Felix Kling
– Felix Kling, Commented Mar 1, 2019 at 0:16

Felix Kling · Accepted Answer · 2019-03-01 00:25:05Z

You need to process and mutate every element in the array (assuming in-place mutation is fine), so .filter is not suitable here, since it usually just "removes" elements.

A general approach would be: Iterate over every element of the array and keep a cod => previous item map to keep track of the previous item with the same cod value. Set val of the current item to true, set val of the previous item with the same cod value to false, update tracking entry for current item's cod value.

Example:

const data = [{
   id: 1,
   cod: '123',
   val: true      
},
{
   id: 2,
   cod: '123',
   val: true      
},
{
   id: 3,
   cod: '123',
   val: true      
},
{
   id: 4,
   cod: '456',
   val: true      
},
{
   id: 5,
   cod: '456',
   val: true      
}];

const previousCod = new Map();

for (const item of data) {
  if (previousCod.has(item.cod)) {
    previousCod.get(item.cod).val = false;
  }
  // Can be omitted if the initial value is guaranteed to be `true`
  item.val = true;
  previousCod.set(item.cod, item);
}

console.log(data);

Collectives™ on Stack Overflow

Setting property of objects in array

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related