Lodash is trying to be helpful by recursively merging subobjects

This method is like _.assign except that it recursively merges own and inherited enumerable string keyed properties of source objects into the destination object. Source properties that resolve to undefined are skipped if a destination value exists. Array and plain object properties are merged recursively. Other objects and value types are overridden by assignment. Source objects are applied from left to right. Subsequent sources overwrite property assignments of previous sources.

https://lodash.com/docs/4.17.15#merge (emphasis mine)

If you don't want this to happen, just use the standard ES6 spread operator ... or Object.assign

const object1 = { 
  name: 'John',
  age: 23,
  books: ['book1', 'book2']
};

const object2 = { 
  name: 'John',
  age: 23,
  books: ['book3'],
  city: 'London'
};

object3 = {...object1, ...object2};
object4 = Object.assign({}, object1, object2);

console.log(object3);
console.log(object4);

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If both arrays are in the correct order; where each item corresponds to its associated member identifier then you can simply use.

var merge = _.merge(arr1, arr2);

Which is the short version of:

var merge = _.chain(arr1).zip(arr2).map(function(item) {
    return _.merge.apply(null, item);
}).value();

Or, if the data in the arrays is not in any particular order, you can look up the associated item by the member value.

var merge = _.map(arr1, function(item) {
    return _.merge(item, _.find(arr2, { 'member' : item.member }));
});

You can easily convert this to a mixin. See the example below:

_.mixin({
  'mergeByKey' : function(arr1, arr2, key) {
    var criteria = {};
    criteria[key] = null;
    return _.map(arr1, function(item) {
      criteria[key] = item[key];
      return _.merge(item, _.find(arr2, criteria));
    });
  }
});

var arr1 = [{
  "member": 'ObjectId("57989cbe54cf5d2ce83ff9d6")',
  "bank": 'ObjectId("575b052ca6f66a5732749ecc")',
  "country": 'ObjectId("575b0523a6f66a5732749ecb")'
}, {
  "member": 'ObjectId("57989cbe54cf5d2ce83ff9d8")',
  "bank": 'ObjectId("575b052ca6f66a5732749ecc")',
  "country": 'ObjectId("575b0523a6f66a5732749ecb")'
}];

var arr2 = [{
  "member": 'ObjectId("57989cbe54cf5d2ce83ff9d8")',
  "name": 'yyyyyyyyyy',
  "age": 26
}, {
  "member": 'ObjectId("57989cbe54cf5d2ce83ff9d6")',
  "name": 'xxxxxx',
  "age": 25
}];

var arr3 = _.mergeByKey(arr1, arr2, 'member');

document.body.innerHTML = JSON.stringify(arr3, null, 4);

body { font-family: monospace; white-space: pre; }

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You can get the result using merge and keyBy lodash functions.

var array1 = [{id:1, name:'doc1'}, {id:2, name:'doc2'}];
var array2 = [{id:1, name:'doc1'}, {id:3, name:'doc3'}, {id:4, name:'doc4'}];

var merged = _.merge(_.keyBy(array1, 'id'), _.keyBy(array2, 'id'));
var values = _.values(merged);
console.log(values);

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Second highest voted answer doesn't do proper merge. If second array contains an unique property, it is not taken into account.

This approach does a proper merge.

Lodash

var a = [
  { userId:"p1", item:1},
  { userId:"p2", item:2},
  { userId:"p3", item:4}
];

var b = [
  { userId:"p1", profile:1},
  { userId:"p2", profile:2},
  { userId:"p4", profile:4}
];
var merged = _.merge(_.keyBy(a, 'userId'), _.keyBy(b, 'userId'));
var values = _.values(merged);
console.log(values);

<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.10/lodash.min.js"></script>

ES6+

// from https://stackoverflow.com/a/34749873/80766
const mergeDeep = (target, ...sources) => {
  if (!sources.length) return target;
  const source = sources.shift();

  if (target instanceof Object && source instanceof Object) {
    for (const key in source) {
      if (source[key] instanceof Object) {
        if (!target[key]) Object.assign(target, { [key]: {} });
        mergeDeep(target[key], source[key]);
      } else {
        Object.assign(target, { [key]: source[key] });
      }
    }
  }

  return mergeDeep(target, ...sources);
}

const a = [
  { userId:"p1", item:1},
  { userId:"p2", item:2},
  { userId:"p3", item:4}
];

const b = [
  { userId:"p1", profile:1},
  { userId:"p2", profile:2},
  { userId:"p4", profile:4}
];


const aKeyed = a.reduce((acc, cur) => ({ ...acc, [cur.userId]: cur }), {});
const bKeyed = b.reduce((acc, cur) => ({ ...acc, [cur.userId]: cur }), {});
const merged = mergeDeep(aKeyed, bKeyed);
const values = Object.values(merged);
console.log(values);