Proxy API usage scenarios —— Builder API

builder.js

function Builder(typeOrTemplate, template) {
  let type;
  if (typeOrTemplate instanceof Function) {
    type = typeOrTemplate;
  } else {
    template = typeOrTemplate;
  }
  const built = template ? Object.assign({}, template) : {};
  const builder = new Proxy(
    {},
    {
      get(target, prop) {
        if ("build" === prop) {
          if (type) {
            // A class name (identified by the constructor) was passed. Instantiate it with props.
            const obj = new type();
            return () => Object.assign(obj, Object.assign({}, built));
          } else {
            // No type information - just return the object.
            return () => built;
          }
        }
        return (...args) => {
          // If no arguments passed return current value.
          if (0 === args.length) {
            return built[prop.toString()];
          }
          built[prop.toString()] = args[0];
          return builder;
        };
      },
    }
  );
  return builder;
}