Find duplicate value in array of objects and merge some duplicate key and value

You can use Array.prototype.reduce() to reduce your array as you need.

Duplicate item can be merged using Object.assign().

var array = [
    { 'id_0': 356, 'name': 'India', 'key1': 150 },
    { 'id_0': 356, 'name': 'India', 'key2': 200 }
];

var result = array.reduce(function(prev, item) {
    var newItem = prev.find(function(i) {
        return i.id_0 === item.id_0; 
    });
    if (newItem) {
        Object.assign(newItem, item);
    } else {
        prev.push(item);
    }
    return prev;
}, []);

console.log(result);

Object.assign is part of ES6. If it does not work for you, just replace it with:

for (var attrname in item) {
    newItem[attrname] = item[attrname];
};

Try this fiddle

var array = [
 {
  "id_0":356,
  "name":"India",
  "key1":150
 },
 {
  "id_0":356,
  "name":"India",
  "key2":200
 },
 {
  "id_0":356,
  "name2":"china",
  "key2":200
 }
]
function mergeArray( arr )
{
   var outputObj = {};
   for ( var counter = 0; counter < arr.length; counter++ )
   {
      var obj = arr[ counter ];
      for( var key in obj )
      {
         if ( !outputObj[ key ] )
         {
           outputObj[ key ] = obj[ key ];
         }
      }
   }
   return outputObj;
}
console.log( mergeArray( array ) );

Edited the fiddle to suit your 'UPDATED' requirement

var array = [
 {
  "id_0":356,
  "name":"India",
  "key1":150
 },
 {
  "id_0":356,
  "name":"India",
  "key2":200
 },
 {
  "id_0":400,
  "name2":"china",
  "key2":200
 },
 {
  "id_0":400,
  "name2":"china",
  "key2":200
 }
]
function mergeArray( arr )
{
   var outputObj = {};
   for ( var counter = 0; counter < arr.length; counter++ )
   {
      var obj = arr[ counter ];
      for( var key in obj )
      {
         if ( !outputObj[ key ] )
         {
           outputObj[ key ] = obj[ key ];
         }
      }
   }
   return outputObj;
}
function collateArray( arr )
{
   var outputObj = {};
   var result = [];
   for ( var counter = 0; counter < arr.length; counter++ )
   {
      var obj = arr[ counter ];
      var id_0value = obj[ "id_0" ];
      if ( !outputObj[ id_0value ] )
      {
        outputObj[ id_0value ] = [];
      }
      outputObj[ id_0value ].push( obj );
   }
   console.log( outputObj );
   for ( var key in outputObj )
   {
      result.push( mergeArray( outputObj[ key ] ) );
   }
   return result;

}
console.log( collateArray( array ) );

Use an temp Object as a map that stores key => item can make the time complexity to O(n):

var arr = [
 {
  "id_0":356,
  "name":"India",
  "key1":150
 },
 {
  "id_0":356,
  "name":"India",
  "key2":200
 },
 {
  "id_0":748,
  "name":"Swaziland",
  "key1":140
 },
 {
  "id_0":748,
  "name":"Swaziland",
  "key2":180
 }
];

// items:            the array to be merged to unique.
// attrName:    the attribute's name that is for distinguish,
// isOverwrite:decides whether the value will be overwrite by later ones or not. Default to false. 
function getUnique(items, attrName, isOverwrite) {
  // Map to keep reference to objects by its id.
  var store = {};
  // The array for output result.
  var result = [];
  
  isOverwrite = !!isOverwrite;
  
  items.forEach(function(item) {
    var id = item[attrName];
    var key;
    
    // Try to fetch item by id from store.
    var target = store[id];
    
    // If target item exist in store, its dulplicated, we need to merge it.
    if (typeof target !== 'undefined') {
      // If it's presented, we need to merge it into existing one.
      for (key in item) {
        if (!item.hasOwnProperty(key)) {
          continue;
        }
        
        // If isOverwrite is true, always use the newest value, otherwise,
        // we only apply values that are not exist in target yet.
        if (isOverwrite || typeof target[key] === 'undefined') {
          target[key] = item[key];
        }
      }
    } else {
      // If its not in store yet, put it a clone into result, and to map for 
      // later reference.
      var clone = {};
      for (key in item) {
        if (item.hasOwnProperty(key)) {
          clone[key] = item[key];
        }        
      }
      store[id] = clone;      
      // Also put it into the result array.
      result.push(clone);
   }
  });

  return result;
}

console.log(getUnique(arr, 'id_0'));