Typescript array of objects check for duplicates

Question

I have requirement where I need to check duplicates for object items.

In the below Array of object, I need to check for "empno" OR "extension" and If there are any duplicates then I need to throw an error.

I have tried this Stack Overflow link but it didn't work for me.

[
    {
        "id": 269,
        "empno": "34567",
        "extension": 345
    },
    {
        "id": 269,
        "empno": "34568",
        "extension": 346
    },
    {
        "id": 269,
        "empno": "34569",
        "extension": 345
    },
    {
        "id": 269,
        "empno": "34567",
        "extension": 345
    }
]

Robby Cornelissen · Accepted Answer · 2020-05-28 04:00:10Z

You could build a Set to allow for efficient lookups of existing values:

const hasDuplicates = (data) => {
  const set = new Set();

  for (let {empno, extension} of data) {
    const key = `${empno}/${extension}`;
    if (set.has(key)) {
      return true;
    }

    set.add(key);
  }

  return false;
};

Full snippet:

const data = [{
    "id": 269,
    "empno": "34567",
    "extension": 345
  },
  {
    "id": 269,
    "empno": "34568",
    "extension": 346
  },
  {
    "id": 269,
    "empno": "34569",
    "extension": 345
  },
  {
    "id": 269,
    "empno": "34567",
    "extension": 345
  }
];

const hasDuplicates = (data) => {
  const set = new Set();
  
  for (let {empno, extension} of data) {
    const key = `${empno}/${extension}`;
    if (set.has(key)) {
      return true;
    }
    
    set.add(key);
  }
  
  return false;
};

console.log(hasDuplicates(data));

If you want something less verbose at the cost of processing the entire array even if duplicates have already been found, you can reduce to a Set and compare its size with the original array:

const hasDuplicates = (data) => data.length !== data.reduce((a, {empno, extension}) => a.add(`${empno}/${extension}`), new Set).size;

Full snippet:

const data = [{
    "id": 269,
    "empno": "34567",
    "extension": 345
  },
  {
    "id": 269,
    "empno": "34568",
    "extension": 346
  },
  {
    "id": 269,
    "empno": "34569",
    "extension": 345
  },
  {
    "id": 269,
    "empno": "34567",
    "extension": 345
  }
];

const hasDuplicates = (data) => data.length !== data.reduce((a, {empno, extension}) => a.add(`${empno}/${extension}`), new Set).size;

console.log(hasDuplicates(data));

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