Deep merging objects in JavaScript is a bit tricky. Here’s one solution; there are many many others. While this was fun to write, you’ll probably want to go use the merge function in lodash instead. None the less here’s some code golf

function merge(...sources) {
  return sources.reduce((target, source) => {  // enumerate the sources
    return Object.entries(source).reduce((result, [key, value]) => {  // enumerate the properties
      if(Array.isArray(value)) {
        if(Array.isArray(result[key])) {
          result[key] = result[key].concat(value); // got two arrays, merge them
        } else {
          result[key] = value;  // clobber it, either undefined or not an array
        }
      } else if(value === Object(value)) {
        if(result[key] === Object(result[key])) {
          result[key] = merge(result[key], value); // two objects, recurse and merge them
        } else {
          result[key] = merge(value); // recurse and clobber it, either undefined or not an object
        }
      } else {
        result[key] = value;  // must be a primitive, clobber it
      }
      return result;
    }, target);
  }, {});
}

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