@BTMPL since you've already created the PR I thought about, may I ask you to expand it a little bit? For the sake of demonstrating default parameters, I suggest to remove the fields title and buttonText from menuConfig completely, and change body and cancellable values to something different from default values. Because even with your fix, the first two will become null, and the last two remain unchanged.

I doubt that null values were intentional.

@stasgavrylov I've provided a more detailed example, please review:

https://github.com/ryanmcdermott/clean-code-javascript/pull/9/files?short_path=04c6e90#diff-04c6e90faac2675aa89e2176d2eec7d8

much better, imo. although, I'd introduce new variable for assignment operation result to avoid parameter mutation.

Thank you.

There is no (apparent) downside of parameter mutation, other than if you need to access the original value for some reason. It wont mutate menuConfig.

Thank you so much for looking at this! I've taken your PR as inspiration for this fixed and fully fleshed out example: d00ca4c

In your new example, you introduce a dummy object to copy all of the params to.

  config = Object.assign({}, {
    title: 'Foo',
    body: 'Bar',
    buttonText: 'Baz',
    cancellable: true
  }, config);

This adds unnecessary performance overhead. You're instantiating two object literals, and performing 2 copy operations: first from the defaults to the dummy object, then from the config to the dummy object.

A fresh defaults object is instantiated every time this code is run so mutating it is a non issue. You only need to copy the config to the defaults object. There's no need for the dummy object.