Q:

Javascript merge/reduce same multi dimensional objects

based on my question: https://stackoverflow.com/a/40661953/2392461, i open a new question with sample data.
I want to merge/reduce this:
var array = [{
  'key1': {
    'key11': 0,
    'key12': 1
  },
  'key2': 0,
  'key3': {
    'key31': [1, 2],
    'key32': {
        'key321': 3,
        'key322': [1, 2]
    }
  },
  'key4': 'test'
  }, {
  'key1': {
    'key11': 1,
    'key12': 9
  },
  'key2': 2,
  'key3': {
    'key31': [4, 3],
    'key32': {
        'key321': 6,
        'key322': [8, 9]
    }
  },
  'key4': 'test'
  }, {
  'key1': {
    'key11': 3,
    'key12': 4
  },
  'key2': 7,
  'key3': {
    'key31': [3, 2],
    'key32': {
        'key321': 6,
        'key322': [7, 8]
    }
  },
  'key4': 'test'
}];

to this:
{
  'key1': {
    'key11': [0, 1, 3],
    'key12': [1, 9, 4]
  },
  'key2': [0, 2, 7],
  'key3': {
    'key31': [[1, 2], [4, 3], [3, 2]],
    'key32': {
        'key321': [3, 6, 6],
        'key322': [[1, 2], [8, 9], [7, 8]]
    }
  },
  'key4': 'test'
}

the reduce function (https://stackoverflow.com/a/40668315/2392461) from @stasovlas with lodash looks good but i need to go deeper in the object.
_.reduce(data, function(result, item) {
    var added = _.find(result, {
        key4: item.key4
    });
    if (_.isObject(added)) {
        //!! better to merge with new object and add result to array again to avoid mutable
        added = _.mergeWith(added, item, function(addedVal, itemVal, key) {
            if (key === 'key4') {
                return addedVal;
            }
            return _.concat(addedVal, itemVal);
        });
        return result;
    }
    return _.concat(result, item);
}, []);

result is here a merged object only in layer 1 of the object.
[ { key1: [ [Object], [Object], [Object] ],
    key2: [ 0, 2, 7 ],
    key3: [ [Object], [Object], [Object] ],
    key4: 1 } ]

i have no idea how to get my result. i think i have to iterate each object n times. n is the depth of the object.
is this the right way or is it easier than i think?
greetings mok

A:

You could merge each level by iterating the entries of the source object and create either a new object or an array for adding the result. Proceed with child objects.
key4 gets a special treatment.

function merge(target, source) {
    var singleKey = 'key4';
    Object.entries(source).forEach(([key, value]) => {
        if (value && typeof value === 'object' && !Array.isArray(value)) {
            merge(target[key] = target[key] || {}, value);
            return;
        }
        if (key === singleKey) {
            target[key] = value;
            return;
        }
        target[key] = target[key] || [];
        target[key].push(value);
    });
    return target;
}

var array = [{ key1: { key11: 0, key12: 1 }, key2: 0, key3: { key31: [1, 2], key32: { key321: 3, key322: [1, 2] } }, key4: "test" }, { key1: { key11: 1, key12: 9 }, key2: 2, key3: { key31: [4, 3], key32: { key321: 6, key322: [8, 9] } }, key4: "test" }, { key1: { key11: 3, key12: 4 }, key2: 7, key3: { key31: [3, 2], key32: { key321: 6, key322: [7, 8] } }, key4: "test" }],
    result = array.reduce(merge, {});

console.log(result);
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