Lodash

assignWith

Object
since 4.0.0

Returns

Object `object`.

This method is like _.assign except that it accepts customizer which is invoked to produce the assigned values. If customizer returns undefined, assignment is handled by the method instead. The customizer is invoked with five arguments: (objValue, srcValue, key, object, source).

Note: This method mutates object.

Example of Lodash _.assignWith

function customizer(objValue, srcValue) {
  return _.isUndefined(objValue) ? srcValue : objValue;
}

var defaults = _.partialRight(_.assignWith, customizer);

defaults({ 'a': 1 }, { 'b': 2 }, { 'a': 3 });
// => { 'a': 1, 'b': 2 }