var arvConcat = (r) =>{
    var rr=[],or=[];
    for(let ri of r){        
        if(typeof rr[ri.key] === 'undefined')rr[ri.key]=[];
        rr[ri.key].push(ri.value);
    }
    for(let ri in rr){
        or.push({key:ri,value:rr[ri]});
    }
    return or;
}

let r = [{
  key: '1',
  value: 'VALUE1'
}, {
  key: '2',
  value: 'VALUE2'
}, {
  key: '1',
  value: 'VALUE3'
}, {
  key: '3',
  value: 'VALUE4'
}];

let newR = arvConcat(r);

console.log(newR);

let array = [{
    key: '1',
    value: 'VALUE1',
  },
  {
    key: '2',
    value: 'VALUE2',
  },
  {
    key: '1',
    value: 'VALUE3',
  },
  {
    key: '3',
    value: 'VALUE4',
  },
];

let i = 0;
while (i < array.length) {
  if (i + 1 <= array.length) {
    for (let x = i + 1; x < array.length; x++) {
      if (array[i].key === array[x].key) {
        array[i].value = `${array[x].value} ${array[i].value}`;
        array.splice(x, 1);
      }
    }
    i++
  }
}
console.log(array);

//Input:
const array = [{
  key: '1',
  value: 'VALUE1'
}, {
  key: '2',
  value: 'VALUE2'
}, {
  key: '1',
  value: 'VALUE3'
}, {
  key: '3',
  value: 'VALUE4'
}]

const result = Object.values(array.reduce((a,o) => {
      a[o.key] = a[o.key] || {'key': o.key, 'value' : []};
      a[o.key]['value'].push(o.value);
      return a;
    }, {}));

console.log(result);