Remove duplicates from array when two values are the same

Question

My json array with duplicates looks like:

var list = [
  {"id":"123","class": "Math K", "subject": "Counting & Cardinality"},
  {"id":"124","class": "Math K", "subject": "Counting & Cardinality"},
  {"id":"125","class": "Math K", "subject": "Counting & Cardinality"},
  {"id":"126","class": "Math K", "subject": "Counting & Cardinality"},
  {"id":"127","class": "Math K", "subject": "Geometry"},
  {"id":"128","class": "Math 1", "subject": "Counting & Cardinality"},
  {"id":"129","class": "Math 1", "subject": "Counting & Cardinality"},
  {"id":"120","class": "Math 1", "subject": "Orders of Operation"},
  {"id":"121","class": "Math 2", "subject": "Geometry"},
  {"id":"122","class": "Math 2", "subject": "Geometry"}
];

I need to remove duplicates with the same Class and Subject (e.g Math K and Counting & Cardinality or Math 2 and Geometry) but keep IDs for the first record, so it should look like:

var newList = [
  {"id":"123","class": "Math K", "subject": "Counting & Cardinality"},
  {"id":"127","class": "Math K", "subject": "Geometry"},
  {"id":"128","class": "Math 1", "subject": "Counting & Cardinality"},
  {"id":"120","class": "Math 1", "subject": "Orders of Operation"},
  {"id":"121","class": "Math 2", "subject": "Geometry"},
];

I have tried this code:

var newList = [];
for( var class in list ) {
  for( var subject in list[class] ) {
      outputList.push({ id: id, class: class, subject: subject });
  }
}
JSON.stringify( newList, null, 4 );

But it doesn't bring the results. I would really appreciate your help with tweaking my code. Thank you.

JSON is a textual notation for data exchange. (More here.) If you're dealing with JavaScript source code, and not dealing with a string, you're not dealing with JSON. — T.J. Crowder
– T.J. Crowder, Commented Aug 17, 2021 at 11:13
With all due respect, nothing in the code you've shown does anything that would even try to do what you've described (and should throw an error when run, as id is undeclared but you try to read its value). Your best bet here is to do your research, search for related topics on SO, and give it a go. If you get stuck and can't get unstuck after doing more research and searching, post a minimal reproducible example of your attempt and say specifically where you're stuck. People will be glad to help. — T.J. Crowder
– T.J. Crowder, Commented Aug 17, 2021 at 11:14
Does this answer your question? Remove Duplicate objects from JSON Array — LeeLenalee
– LeeLenalee, Commented Aug 17, 2021 at 11:15
hi @LeeLenalee no, I used the example from that question, but it doesn't work with my IDs, which needs to be kept as the first one from each duplicate. — qqruza
– qqruza, Commented Aug 17, 2021 at 11:17
@qqruza then update the example to use your IDs? At the moment that question and yours are very similar — evolutionxbox
– evolutionxbox, Commented Aug 17, 2021 at 11:23

Wimanicesir · Accepted Answer · 2021-08-17 11:31:42Z

var list = [
  {"id":"123","class": "Math K", "subject": "Counting & Cardinality"},
  {"id":"124","class": "Math K", "subject": "Counting & Cardinality"},
  {"id":"125","class": "Math K", "subject": "Counting & Cardinality"},
  {"id":"126","class": "Math K", "subject": "Counting & Cardinality"},
  {"id":"127","class": "Math K", "subject": "Geometry"},
  {"id":"128","class": "Math 1", "subject": "Counting & Cardinality"},
  {"id":"129","class": "Math 1", "subject": "Counting & Cardinality"},
  {"id":"120","class": "Math 1", "subject": "Orders of Operation"},
  {"id":"121","class": "Math 2", "subject": "Geometry"},
  {"id":"122","class": "Math 2", "subject": "Geometry"}
];

var unique = [];
var result = [];
list.forEach(el => {
  // Let's create a string that is a combination of the class and subject
  var check = el.class + el.subject;
  // If its not in the unique array, let's add it
  if (unique.indexOf(check) < 0) {
    result.push(el);
    unique.push(check);
  }
})

console.log(result)

ikhvjs · Accepted Answer · 2021-08-17 12:04:20Z

Another way to do it is to remove duplicates with a reduce().

The list.reduce() returns a object with keys of ${b.class}_${b.subject} and the values with the object looks like below:

{
  "Math K_Counting & Cardinality": {
    "id": "123",
    "class": "Math K",
    "subject": "Counting & Cardinality"
  },
  "Math K_Geometry": { "id": "127", "class": "Math K", "subject": "Geometry" },
  "Math 1_Counting & Cardinality": {
    "id": "128",
    "class": "Math 1",
    "subject": "Counting & Cardinality"
  },
  "Math 1_Orders of Operation": {
    "id": "120",
    "class": "Math 1",
    "subject": "Orders of Operation"
  },
  "Math 2_Geometry": { "id": "121", "class": "Math 2", "subject": "Geometry" }
}

Then extract the values of list.reduce() to get the result by Object.values()

const list = [
  { id: "123", class: "Math K", subject: "Counting & Cardinality" },
  { id: "124", class: "Math K", subject: "Counting & Cardinality" },
  { id: "125", class: "Math K", subject: "Counting & Cardinality" },
  { id: "126", class: "Math K", subject: "Counting & Cardinality" },
  { id: "127", class: "Math K", subject: "Geometry" },
  { id: "128", class: "Math 1", subject: "Counting & Cardinality" },
  { id: "129", class: "Math 1", subject: "Counting & Cardinality" },
  { id: "120", class: "Math 1", subject: "Orders of Operation" },
  { id: "121", class: "Math 2", subject: "Geometry" },
  { id: "122", class: "Math 2", subject: "Geometry" },
];

const output = Object.values(
  list.reduce((acc, b) => {
    if (!acc[`${b.class}_${b.subject}`]) acc[`${b.class}_${b.subject}`] = b;
    return acc;
  }, {})
);

console.log(output);

Carsten Massmann · Accepted Answer · 2021-08-17 12:42:00Z

Just for fun: a one-line-solution:

var list = [
  {"id":"123","class": "Math K", "subject": "Counting & Cardinality"},
  {"id":"124","class": "Math K", "subject": "Counting & Cardinality"},
  {"id":"125","class": "Math K", "subject": "Counting & Cardinality"},
  {"id":"126","class": "Math K", "subject": "Counting & Cardinality"},
  {"id":"127","class": "Math K", "subject": "Geometry"},
  {"id":"128","class": "Math 1", "subject": "Counting & Cardinality"},
  {"id":"129","class": "Math 1", "subject": "Counting & Cardinality"},
  {"id":"120","class": "Math 1", "subject": "Orders of Operation"},
  {"id":"121","class": "Math 2", "subject": "Geometry"},
  {"id":"122","class": "Math 2", "subject": "Geometry"}
];

// a.u: object with unique keys, a.c: array with the collection of first unique values:
res=list.reduce((a,el,k)=>(a.u[k=el.class+'_'+el.subject] || (a.u[k]=1) && a.c.push(el),a),{u:{},c:[]}).c;

console.log(res)

The script makes use of the fact that logical expressions are evaluated in a "lazy fashion", i. e. if the first operand of an or (||) expression evaluates to true then the second operand will never be evaluated.

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