Here is one option:-

var array = [{
  name: "foo1",
  value: "val1"
}, {
  name: "foo1",
  value: ["val2", "val3"]
}, {
  name: "foo2",
  value: "val4"
}];

var output = [];

array.forEach(function(item) {
  var existing = output.filter(function(v, i) {
    return v.name == item.name;
  });
  if (existing.length) {
    var existingIndex = output.indexOf(existing[0]);
    output[existingIndex].value = output[existingIndex].value.concat(item.value);
  } else {
    if (typeof item.value == 'string')
      item.value = [item.value];
    output.push(item);
  }
});

console.dir(output);

You could use reduce with Object.keys to do that.

var arr = [
    {id: 1, tech_id:11, action: 'swim'},
    {id: 2, tech_id:11, action: 'run'},
    {id: 3, tech_id:22, action: 'climb'},
    {id: 4, tech_id:22, action: 'swim'},
    {id: 5, tech_id:11, action: 'jump'},
]

let mergeObj = arr.reduce((p, c) => {
    const {tech_id, ...otherData} = c;

    if (!(tech_id in p)) {
        p[tech_id] = {
            data: []
        }
    }
    
    p[tech_id].data.push(otherData)
    return p
}, {})

mergeObj = Object.keys(mergeObj).map(key => {
    return {
        tech_id: key,
        data: mergeObj[key].data
    }
})

console.log(mergeObj);

I'd use a temporary object to:

• Loop over array
• For the current object;
  • merge ({...obj1, ...obj2}) current object to res on the ts key
• Use Object.values to convert it to the desired array

const array = [{ts: 1596232800, v1: 1}, {ts: 1596232860, v1: 2}, {ts: 1596232920, v1: 3}, {ts: 1596232800, b2: 10}, {ts: 1596232860, b2: 11}, {ts: 1596232920, b2: 12}, {ts: 1596232800, c3: 20}, {ts: 1596232860, c3: 21}, {ts: 1596232920, c3: 22} ];

let res = {};
array.forEach(a => res[a.ts] = {...res[a.ts], ...a});
res = Object.values(res);

console.log(res);

[
  {
    "ts": 1596232800,
    "v1": 1,
    "b2": 10,
    "c3": 20
  },
  {
    "ts": 1596232860,
    "v1": 2,
    "b2": 11,
    "c3": 21
  },
  {
    "ts": 1596232920,
    "v1": 3,
    "b2": 12,
    "c3": 22
  }
]

You can use Array.prototype.reduce:

const members = [
  {
    groupId: 1,
    name: 'test group',
    userId: 2,
    first_name: 'Ross',
    last_name: 'Geller',
    joinedDate: "2022-01-13T13:25:43.510Z",
    leftDate: "2023-02-24T19:07:11.110Z"   
  },
  {
    groupId: 12,
    name: 'test group 2',
    userId: 2,
    first_name: 'Ross',
    last_name: 'Geller',
    joinedDate: "2022-01-13T00:00:00.000Z",
    leftDate: null
  },
  {
    groupId: 12,
    name: 'test group 2',
    userId: 6,
    first_name: 'Joey',
    last_name: 'Tribbiani',
    joinedDate: "2022-01-13T00:00:00.000Z",
    leftDate: null
  }
];

const grouped = members.reduce((acc, { groupId, name, ...member }) => {
  const existingGroup = acc.find(group => group.groupId === groupId);
  if (existingGroup)
    existingGroup.members.push(member);
  else
    acc.push({
      groupId,
      name,
      members: [
        member
      ]
    });
  return acc;
}, []);

console.log(grouped);

var jsonData = [{
    Question: "Was the training useful?",
    QuestionId: 1,
    myData: [{
      name: 'No',
      value: 1
    }]
  },
  {
    Question: "Was the training useful?",
    QuestionId: 1,
    myData: [{
      name: 'Yes',
      value: 1
    }]
  }
];

const result = Object.values(jsonData.reduce((acc, obj) => {
  if (!acc[obj.QuestionId]) {
    acc[obj.QuestionId] = obj;
  } else {
    acc[obj.QuestionId].myData = acc[obj.QuestionId].myData ?? []; // if myData is null or undefined, set it to an empty array
    acc[obj.QuestionId].myData = acc[obj.QuestionId].myData.concat(obj.myData);
  }
  return acc;
}, {}));

console.log(result);

var obj1 = {name: "John", hobby: "Football"}

var obj2 = {hobby: "Basketball"}
var obj3 = {hobby: "Tennis"}

const merge = (obj1, obj2) => {
    Object.keys(obj1).forEach(key => { if (obj2[key]) obj1[key] = [...[obj1[key]], obj2[key]].flat() })
}

merge(obj1, obj2)
merge(obj1, obj3)

console.log(obj1)

You can simply use Array.reduce() to create a map, grouped by name and Object.values() on the map will give your the desired result:

let arr = [ { "name": "Blah", "description": "This is a description" }, { "name": "Foo", "description": "This is a description" }, { "name": "Bar", "description": "This is a description" }, { "name": "Blah", "description": "This is a description" }, { "name": "Test", "description": "This is a description" } ];
let result = Object.values(arr.reduce((a, {name,description})=>{
  if(a[name])
    a[name].description += "\n" + description;
   else
    a[name] = Object.assign({},{name,description});
   return a;
},{}));
console.log(result);

You can do it with help of reduce

let rows = [{ row1: [{ val: "row1-val1" }, { val: "row1-val2" }] },{ row1: [{ val: "row1-val3" }, { val: "row1-val4" }] },{ row1: [{ val: "row1-val5" }, { val: "row1-val6" }] },{ parag: { text: "sample test" } },{ row2: [{ val: "row2-val1" }, { val: "row2-val2" }] },{ row2: [{ val: "row2-val3" }, { val: "row2-val4" }] },{ row2: [{ val: "row2-val5" }, { val: "row2-val6" }] }];

let op = rows.reduce((o,c)=>{
  let key = Object.keys(c)[0];
  if(o[key]){
    o[key].push(...c[key]);
  } else {
    o[key] = c[key];
  }
  return o;
},{})

console.log(op)

You're example includes invalid syntax for an array.

["topicName":"topic check check","topicName":"topic check popup"]

This could be an array of objects:

[{"topicName":"topic check check"},{"topicName":"topic check popup"}]

Or more likely (how my example is based):

topicName: ["topic check check", "topic check popup"]

Because it is not a one-to-one input to output, it is a good use of Array.prototype.reduce.

list.reduce((acc, row) => {
  const existingSel = acc.find(e => e.selectionId === row.selectionId);
  
  // If we don't have an entry, make one.
  if (!existingSel) {
    // Use expansion of row to avoid mutating source objects
    return [ ...acc, { ...row}];
  }
  
  if (Array.isArray(existingSel.topicName)) {
    // if the topicName is an array, add to it.
    existingSel.topicName.push(row.topicName);
  } else {
    // Otherwise, make it an array with the two options.
    existingSel.topicName = [existingSel.topicName, row.topicName];
  }
  return acc;
}, []);

You can use reduce for that and iterate over the object keys of each object:

const res = array.reduce((a, b) => {
    Object.keys(b).forEach(k => {
        a[k] = (a[k] || 0) + b[k];
    });
    return a;
}, Object.create(null));

console.log(res);

<script>
const array = [
    {
        RT: 0.3396966806905289,
        Real: 0.03721461991442617,
        fired: 0.26138293747172237,
        'Real Madrid': 0.15767570563383954,
        Madrid: 0.2771992640023346,
        Zidane: 0.3218878635800967,
        'Florentino Perez': 0.31535141126767907,
        leave: 0.1632670522868711,
        manager: 0.12690962022792524,
        tancredipalmeri: 0.3218878635800967,
        Perez: 0.4543985280046692,
        Florentino: 0.44189852800466917
    },
    {
        Cannavaro: 0.2709492640023346,
        Ashley: 0.2771992640023346,
        'Ashley Cole': 0.15767570563383954,
        Casillas: 0.3896966806905289,
        Ronaldo: 0.22094926400233458,
        Zidane: 0.22094926400233458,
        Messi: 0.22094926400233458,
        Lampard: 0.22094926400233458,
        Iniesta: 0.22094926400233458,
        Terry: 0.2771992640023346,
        Henry: 0.19890866571173632,
        Cole: 0.22094926400233458,
        Ivanovic: 0.3218878635800967
    },
    {
        RT: 0.3396966806905289,
        effleurer: 0.22094926400233458,
        'n\'a': 0.2709492640023346,
        top: 0.13189246756926315,
        Zidane: 0.2709492640023346,
        entraineur: 0.22094926400233458,
        monde: 0.22094926400233458,
        au: 0.2271992640023346,
        'au monde': 0.20434237230050623,
        Mvssi10: 0.3218878635800967,
        JAMAIS: 0.2709492640023346
    },
    {
        RT: 0.3396966806905289,
        Real: 0.10790900513119943,
        FrenchRMCF: 0.3218878635800967,
        en: 0.2271992640023346,
        histoire: 0.2709492640023346,
        coach: 0.2271992640023346,
        au: 0.2271992640023346,
        Zidane: 0.22094926400233458,
        'en tant': 0.15767570563383954,
        tant: 0.22094926400233458
    }
]
</script>