Define public API

Thank you all for this valuable resource. I was catching up on current practices with respect to "private" properties in JavaScript when I came upon the discussion in #490. It seems to me that the definition of public API used by this style guide is stricter than what SemVer describes. For example, the guide currently states that

> Although a leading underscore is a common convention to mean “private”, in fact, these properties are fully public, and as such, are part of your public API contract.

So, as far as this guide is concerned, anything which is accessible in a JavaScript library constitutes its public API. On the other hand, SemVer describes public API differently:

> 1. Software using Semantic Versioning MUST declare a public API. This API could be declared in the code itself or exist strictly in documentation. However it is done, it should be precise and comprehensive.

I take this to mean that, as a library author, I am free to explicitly include and exclude properties from my public API, regardless of whether or not they are accessible in my JavaScript library.

So I propose that this guide define public API for itself and explain why it adheres to the definition that it does. I think this would inform developers evaluating the pros and cons of whether or not to adopt or reject a private convention in their projects.

Start by reading the discussion in issue #490 and the guide's current statement that underscore-prefixed properties are public API. Define what this guide means by public API and document why that definition is used, then confirm the guidance addresses the distinction from Semantic Versioning.

Written by the indexing model from the issue text.