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Javascript - How do I prevent repeated items from being added?

I have this code:

for (let item of data) {
     for (let subItem of item.itemsSlider) {
       if (!result[subItem.language]) {
         result[subItem.language] = [];
       }
  
        result[subItem.language].push(subItem)
       
      
     }
   }
   return result

which returns an object like the one in the following image, but adds repeated items. I would like to ask for a solution to prevent repeated items from being added.

and this is the data I am using:

const data = [
  {
    "nid": "61629",
    "title": "K02 - Kiosk Slider - EN",
    "itemsSlider": [
      {
        "nid": "61626",
        "status": "1",
        "titleNode": "01Additional Suitcase - EN",
        "language": "en_US"
      },
      {
        "nid": "61655",
        "status": "1",
        "titleNode": "01Additional Suitcase - EN",
        "language": "en_US"
      }
    ]
  },
  {
    "itemsSlider": [
      {
        "nid": "61699",
        "status": "1",
        "titleNode": "01Additional Suitcase - EN",
        "language": "es_MX"
      },
      {
        "nid": "61628",
        "status": "1",
        "titleNode": "01Additional Suitcase - EN",
        "language": "es_MX"
      }
    ]
  },
  {
    "nid": "61629",
    "title": "K02 - Kiosk Slider - EN",
    "itemsSlider": [
      {
        "nid": "61620",
        "status": "1",
        "titleNode": "01Additional Suitcase - EN",
        "language": "fr_FR"
      },
      {
        "nid": "61620",
        "status": "1",
        "titleNode": "01Additional Suitcase - EN",
        "language": "fr_FR"
      }
    ]
  },
  {
    "nid": "61629",
    "title": "K02 - Kiosk Slider - Mix",
    "itemsSlider": [
      {
        "nid": "61626",
        "status": "1",
        "titleNode": "01Additional Suitcase - EN",
        "language": "es_MX"
      },
      {
        "nid": "61622",
        "status": "1",
        "titleNode": "01Additional Suitcase - EN",
        "language": "en_US"
      }
    ]
  }
]

I could point out all the code I've tried but I think it's useless because they don't work.

Solution:

You're currently not checking if the array already contains the item. You can do that with an if statement. For example:

const data = [
  {
    "nid": "61629",
    "title": "K02 - Kiosk Slider - EN",
    "itemsSlider": [
      {
        "nid": "61626",
        "status": "1",
        "titleNode": "01Additional Suitcase - EN",
        "language": "en_US"
      },
      {
        "nid": "61655",
        "status": "1",
        "titleNode": "01Additional Suitcase - EN",
        "language": "en_US"
      }
    ]
  },
  {
    "itemsSlider": [
      {
        "nid": "61699",
        "status": "1",
        "titleNode": "01Additional Suitcase - EN",
        "language": "es_MX"
      },
      {
        "nid": "61628",
        "status": "1",
        "titleNode": "01Additional Suitcase - EN",
        "language": "es_MX"
      }
    ]
  },
  {
    "nid": "61629",
    "title": "K02 - Kiosk Slider - EN",
    "itemsSlider": [
      {
        "nid": "61620",
        "status": "1",
        "titleNode": "01Additional Suitcase - EN",
        "language": "fr_FR"
      },
      {
        "nid": "61620",
        "status": "1",
        "titleNode": "01Additional Suitcase - EN",
        "language": "fr_FR"
      }
    ]
  },
  {
    "nid": "61629",
    "title": "K02 - Kiosk Slider - Mix",
    "itemsSlider": [
      {
        "nid": "61626",
        "status": "1",
        "titleNode": "01Additional Suitcase - EN",
        "language": "es_MX"
      },
      {
        "nid": "61622",
        "status": "1",
        "titleNode": "01Additional Suitcase - EN",
        "language": "en_US"
      }
    ]
  }
];

const result = {};

for (let item of data) {
  for (let subItem of item.itemsSlider) {
    if (!result[subItem.language]) {
      result[subItem.language] = [];
    }

    // check if the record is already there
    if (!result[subItem.language].find(x => x.nid === subItem.nid)) {
      result[subItem.language].push(subItem)
    }
  }
}

console.log(result);

The condition being checked is up to whatever logic you want to use to define a "duplicate". In this case it's just checking if an element already exists with the same nid value.