'How to modify an array of objects if two following objects have different value?

Consider this array:

const arr = [
  {order: 1, mess:"test1"},
  {order: 2, mess:"test2"},
  {order: 2, mess:"test2"},
  {order: 2, mess:"test2"},
  {order: 3, mess:"test3"},
  {order: 4, mess:"test4"},
]

arr is always sorted by order.

I want to modify it to get:

const arr = [
  {order: 1, mess:"test1"},

  {order: 2, mess:"test2", sep:true},
  {order: 2, mess:"test2"},
  {order: 2, mess:"test2"},

  {order: 3, mess:"test3", sep:true},

  {order: 4, mess:"test4", sep:true},
]

sep must be inserted into an object if it has an order different from the preceding object.

I'm unable to write this function, even if it might be quite simple (I guess!).
At this time, I have this, but it doesn't work at all and I don't understand its behavior.

const arr = [
  {order: 1, mess: "test1"},
  {order: 2, mess: "test2"},
  {order: 2, mess: "test2"},
  {order: 2, mess: "test2"},
  {order: 3, mess: "test3"},
  {order: 4, mess: "test4"},
  ]


arr.forEach(function(value, i) {
  i = Number(i)
  const y = i + 1
  if (arr[i] !== undefined && arr[y] !== undefined) {
    if (arr[i].order === arr[y].order) arr[y].sep = true;
  }
});

console.log(arr)

Do you have any idea? Thanks

Solution 1:^[1]

Simplest change of existing array

const arr = [{ order: 1, mess: "test1" }, { order: 2, mess: "test2" }, { order: 2, mess: "test2" }, { order: 2, mess: "test2" }, { order: 3, mess: "test3" }, { order: 4, mess: "test4" }];

arr.forEach((value, i) => {
  if (i && arr[i-1].order !== arr[i].order) arr[i].sep = true;
});
console.log(arr)

Solution 2:^[2]

You could map a new array and check the item and the predecessor.

const
    data = [{ order: 1, mess: "test1" }, { order: 2, mess: "test2" }, { order: 2, mess: "test2" }, { order: 2, mess: "test2" }, { order: 3, mess: "test3" }, { order: 4, mess: "test4" }],
    result = data.map((o, i, a) => i && a[i - 1].order !== o.order
        ? { ...o, sep: true }
        : o
    );

console.log(result);

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Solution 3:^[3]

Try this

const arr = [
  {order: 1, mess: "test1"},
  {order: 2, mess: "test2"},
  {order: 2, mess: "test2"},
  {order: 2, mess: "test2"},
  {order: 3, mess: "test3"},
  {order: 4, mess: "test4"},
  ]

arr.forEach((obj,index)=>{
  if(index>0 && obj.order != arr[index-1]['order']){
    arr[index]['sep']=true
  }
});
console.log(arr)

Solution 4:^[4]

Changes original array:

arr.forEach((el, idx) => {
  if (idx === 0) {
    el.sep = true;
  } else {
    if (el.order !== arr[idx-1].order) {
      el.sep = true;
    }
  }
})

Solution 5:^[5]

Below is another way to achieve the same result as you want.

I believe a more generalized way.

arr.forEach((obj, index) => {
  
  if(JSON.stringify(obj[index+1]) === JSON.stringify(obj[index+2])) {
    obj[index].sep = true;
  }
})

Solution 6:^[6]

You can copy the array:

let arr2 = [...arr];

Shift it (so the first item is the second one!)

arr2.shift()

Loop over both

const arr = [{ order: 1, mess: "test1" }, { order: 2, mess: "test2" }, { order: 2, mess: "test2" }, { order: 2, mess: "test2" }, { order: 3, mess: "test3" }, { order: 4, mess: "test4" }];
let arr2 = [...arr];
arr2.shift();
const result = [arr[0]];
let sep = true;
for (const [v1, v2] of [arr, arr2]) if (v1.order===v2.order) result.push(v2); else result.push({...v2, sep})
console.log(result)

Sources

This article follows the attribution requirements of Stack Overflow and is licensed under CC BY-SA 3.0.

Source: Stack Overflow

Solution	Source
Solution 1
Solution 2	Nina Scholz
Solution 3	sandarshnaroju
Solution 4	Yuriy Loog
Solution 5	Bilal Ahmad
Solution 6	mplungjan

'How to modify an array of objects if two following objects have different value?

Solution 1:[1]

Solution 2:[2]

Solution 3:[3]

Solution 4:[4]

Solution 5:[5]

Solution 6:[6]