JavaScript Object. Change key name of objects in array

Question

I have an array of objects like this example:

[
  { id: 44, message: 'abcd', name: 'example1' },
  { id: 44, message: 'efgh', name: 'example2' },
  { id: 45, message: 'ijkl', name: 'example3' },
  { id: 45, message: 'mnop', name: 'example4' },
]

As you can see the, id value is repeated. Now i need change key name of "name" to "lastName" only at second repeated object:

[
  { id: 44, message: 'abcd', name: 'example1' },
  { id: 44, message: 'efgh', lastName: 'example2' },
  { id: 45, message: 'ijkl', name: 'example3' },
  { id: 45, message: 'mnop', lastName: 'example4' },
]

Answer 1

You can keep last id track, and modify the return object in map loop

For unsorted list:

 let data = [ { id: 44, message: "abcd", name: "example1" }, { id: 44, message: "efgh", name: "example2" }, { id: 45, message: "ijkl", name: "example3" }, { id: 45, message: "mnop", name: "example4" }, ]; let idMap = new Set(); const result = data.map((item) => { if (idMap.has(item.id)) { const { name: lastName, ...rest } = item; item = {...rest, lastName }; } else { idMap.add(item.id); } return item; }); console.log(result);

Note: data has to be sorted based on id.

 let data = [ { id: 44, message: "abcd", name: "example1" }, { id: 44, message: "efgh", name: "example2" }, { id: 45, message: "ijkl", name: "example3" }, { id: 45, message: "mnop", name: "example4" }, ]; let lastId = ""; const result = data.map((item) => { if (lastId === item.id) { const { name: lastName, ...rest } = item; item = {...rest, lastName }; lastId = ""; } else { lastId = item.id; } return item; }); console.log(result);

Answer 2

You can save the id s of each item and rename the key from name to lastName if the item with this id was already visited.

 let list = [ { id: 44, message: 'abcd', name: 'example1' }, { id: 44, message: 'efgh', name: 'example2' }, { id: 45, message: 'ijkl', name: 'example3' }, { id: 45, message: 'mnop', name: 'example4' }, ]; let checked = []; list.forEach(function (item) { if(checked.includes(item.id)){ item['lastName'] = item['name']; delete item['name']; }else{ checked.push(item.id); } }); console.log(list)