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Christopher Hiller
feebf72eb4
This changes the `fake-driver` implementation to use declaration merging instead of other alternatives to mixins. History: The original code simply assigned the mixin methods to `FakeDriver.prototype` after the `class` definition. In TS, this doesn't fly; you need to be explicit about what class members exist. To get around this, we've tried multiple different approaches: 1. Use a "mixin pattern" as explained in the official TypeScript documentation, which involves _class factories_. This is tedious and difficult to reason about; each mixin must be passed to the next in-line, and we must be explicit about the base class and which _previous_ mixins any given mixin expects. 2. Use public class fields (a recent addition to JS). Basically, these do not work the way you want them to; if a subclass overrides a public field, a method in the superclass which refers to this public field _will use the field as defined in the superclass_--not the subclass. This is different than how methods and accessors work. This inhibits use of inheritance. 3. Add a method _for each method in each mixin_ to the base class, which simply calls `Function.prototype.apply` on the mixin with the context as the base class' `this`. This is too verbose. There may be a better way to do this, but where we've landed is using _declaration merging_ in TypeScript. It allows us to add (but not remove, and I'm not sure about "modify") properties to the "FakeDriver interface", which seems to be the implicit TS interface created by the `class` keyword. Since the declaration merging syntax is not expressible in JavaScript, this requires a `.ts` file. To avoid creating _n_ extra files, it was easier to just make the mixins `.ts` files (you may note that we can mix-and-match JS and TS in our projects at will). The implementation of each mixin uses `Object.assign()` to put new methods on the prototype, which are previously defined in the merged declaration above. Finally, a `satisfies` clause (which is a transient type-check; it does not change any types but it _will_ generate a compiler error) ensures the props being added to the prototype match what we've defined. You'll also note that each method has a `this` parameter, which is not a "real" parameter--it's just an annotation for TS which tells the method what the context is. If we hate that, an alternative is to use `FakeDriver.prototype.method = <method>` instead of the `Object.assign` call. This feels marginally more tedious, but I could go either way.