Group an Array by a field value and push grouped values in an array

Question

I have an array:

let docArray = [ 
    { documentID: 304717110, doublettenID: 111, done: true },
  { documentID: 304717110, doublettenID: 222, done: true },
  { documentID: 304717110, doublettenID: 333, done: true },
  { documentID: 304717110, doublettenID: 444, done: true },
  { documentID: 304717110, doublettenID: 555, done: true },
  { documentID: 304717110, doublettenID: 666, done: true },
  { documentID: 304862280, doublettenID: null, done: true },
  { documentID: 304862281, doublettenID: null, done: true },
  { documentID: 304933685, doublettenID: null, done: true },
  { documentID: 304933871, doublettenID: null, done: true },
  { documentID: 304933870, doublettenID: null, done: true },
  { documentID: 304862282, doublettenID: null, done: true } 
  ]

I would like to group by documentID, and create an Object for each distinct documentID and push the "doublettenID" field values inside an array for each object. How can I easily do this?

just have a look here: https://stackoverflow.com/questions/40774697/how-to-group-an-array-of-objects-by-key — Nina Scholz, Jun 13 '19 at 12:47
I have tried it by myself, but somehow I couldn't figure it out after 4hours, I feel totally stupid sorry ;). I first tried to iterate through all values and create dynamically an object with a key with the id and check if that object with the key is already present or not, but I had the problem, that I could only create the object inside the array for-loop, therefore I had to create a new object all the time instead of a persisted object in the outer-loop, I also tried with .map and .reduce() function but couldn't figure it out... — MMMM, Jun 13 '19 at 12:50

score 1 · Answer 1 · answered Jun 13 '19 at 12:52

You could loop through it and use the documentID as a key, and push to a seperate object.

let docArray = [ 
  { documentID: 304717110, doublettenID: 111, done: true },
  { documentID: 304717110, doublettenID: 222, done: true },
  { documentID: 304717110, doublettenID: 333, done: true },
  { documentID: 304717110, doublettenID: 444, done: true },
  { documentID: 304717110, doublettenID: 555, done: true },
  { documentID: 304717110, doublettenID: 666, done: true },
  { documentID: 304862280, doublettenID: null, done: true },
  { documentID: 304862281, doublettenID: null, done: true },
  { documentID: 304933685, doublettenID: null, done: true },
  { documentID: 304933871, doublettenID: null, done: true },
  { documentID: 304933870, doublettenID: null, done: true },
  { documentID: 304862282, doublettenID: null, done: true } 
];
let groups = {};

docArray.forEach(entry => {
    if (!groups.hasOwnProperty(entry.documentID)) {
 groups[entry.documentID] = [];
    }

    groups[entry.documentID].push(entry.doublettenID);
});

console.log(groups);

if I have an object like this: `{ "304717110": [...]}`, how do I address the object with that dynamically created key as name? — MMMM, Jun 14 '19 at 11:56
```obj["key"]``` or ```obj[variableThatContainsKey]``` instead of ```obj.property```. — Tim VN, Jun 14 '19 at 14:16
well, but the problem is just that I don't know in advance the name of the key... as they are dynamically created based on some other field value, which I dont know beforehand... — MMMM, Jun 14 '19 at 19:14

score 1 · Accepted Answer · answered Jun 13 '19 at 13:04

This is a way to do it, I'm not sure if this is the object format that you need.

let docArray = [ 
    { documentID: 304717110, doublettenID: 111, done: true },
  { documentID: 304717110, doublettenID: 222, done: true },
  { documentID: 304717110, doublettenID: 333, done: true },
  { documentID: 304717110, doublettenID: 444, done: true },
  { documentID: 304717110, doublettenID: 555, done: true },
  { documentID: 304717110, doublettenID: 666, done: true },
  { documentID: 304862280, doublettenID: null, done: true },
  { documentID: 304862281, doublettenID: null, done: true },
  { documentID: 304933685, doublettenID: null, done: true },
  { documentID: 304933871, doublettenID: null, done: true },
  { documentID: 304933870, doublettenID: null, done: true },
  { documentID: 304862282, doublettenID: null, done: true } 
  ]
  
  var ids = []
  var res = []
  
  docArray.forEach(doc => {   
    if (!ids.includes(doc.documentID)) {
     doc.doublettenID = [doc.doublettenID]
    res.push(doc)
      ids.push(doc.documentID)
    } else {
     res.find(d => doc.documentID = d.documentID).doublettenID.push(doc.doublettenID)
    }
 })
  
console.log(res)

what does this syntax do: `doc.doublettenID = [doc.doublettenID]` ? — MMMM, Jun 14 '19 at 12:00
replaces the value of `doc.doublettenID` with a new array containing `doc.doublettenID`, is just to transform the current value to an array containing the current value — Daniel Doblado, Jun 14 '19 at 12:10

score 1 · Answer 3 · answered Jun 13 '19 at 14:45

I would advise you to use Array#reduce function. In my understanding it looks much readable.

let docArray = [{
    documentID: 304717110,
    doublettenID: 111,
    done: true
  },
  {
    documentID: 304717110,
    doublettenID: 222,
    done: true
  },
  {
    documentID: 304717110,
    doublettenID: 333,
    done: true
  },
  {
    documentID: 304717110,
    doublettenID: 444,
    done: true
  },
  {
    documentID: 304717110,
    doublettenID: 555,
    done: true
  },
  {
    documentID: 304717110,
    doublettenID: 666,
    done: true
  },
  {
    documentID: 304862280,
    doublettenID: null,
    done: true
  },
  {
    documentID: 304862281,
    doublettenID: null,
    done: true
  },
  {
    documentID: 304933685,
    doublettenID: null,
    done: true
  },
  {
    documentID: 304933871,
    doublettenID: null,
    done: true
  },
  {
    documentID: 304933870,
    doublettenID: null,
    done: true
  },
  {
    documentID: 304862282,
    doublettenID: null,
    done: true
  }
];

const ar = docArray.reduce((a, c) => {
  debugger;
  let filtered = a.filter(el => el.documentID === c.documentID);
  if (filtered.length > 0) {
    a[a.indexOf(filtered[0])].doublettenID.push(c.doublettenID);
  } else {
    if (c.doublettenID !== null) {
      let originalValue = c.doublettenID;
      c.doublettenID = [];
      c.doublettenID.push(originalValue);
    }

    a.push(c);
  }
  return a;
}, []);

$('#pre').text(JSON.stringify(ar));

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<pre id='pre'></pre>

Group an Array by a field value and push grouped values in an array

3 Answers3