var a = { a: 'oh', b: 'my', c: { a: 'a', b: { c: 'c' } } };

var b = { c: { b: { d: 'a'  }, c: { d: 'd' } } };

var c = _.merge(a, b);

const combineMerge = (target, source, options) => {

    const destination = target.slice()

​

    source.forEach((item, index) => {

        if (typeof destination[index] === 'undefined') {

            destination[index] = options.cloneUnlessOtherwiseSpecified(item, options)

        } else if (options.isMergeableObject(item)) {

            destination[index] = merge(target[index], item, options)

        } else if (target.indexOf(item) === -1) {

            destination.push(item)

        }

    })

    return destination

}

​

var a = { a: 'oh', b: 'my', c: { a: 'a', b: { c: 'c' } } };

var b = { c: { b: { d: 'a'  }, c: { d: 'd' } } };

var c = deepmerge(a, b, { arrayMerge: combineMerge });