_.mergeWith(object, sources, customizer)
functioncustomizer(objValue, srcValue) { if (_.isArray(objValue)) { returnobjValue.concat(srcValue); } } varobject= { 'a': [1], 'b': [2] }; varother= { 'a': [3], 'b': [4] }; _.mergeWith(object, other, customizer); // => { 'a': [1, 3], 'b': [2, 4] }