Type inference for customized Selectors in TypeScript

[AndreyBelym]

Are you requesting a feature or reporting a bug?

Feature

What is the current behavior?

Currently you have to specify interfaces of customized Selector and customized Snapshot: https://github.com/DevExpress/testcafe/blob/master/test/server/data/test-suites/typescript-defs/selectors.ts#L760

What is the expected behavior?

These interfaces can be inferred automatically by Mapped Types from TypeScript 2.1: https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-1.html#mapped-types

Specify your

• testcafe version: 0.16.0-alpha4

[inikulin]

How's that?

[inikulin]

ping @AndreyBelym

[AndreyBelym]

Ok, surprisingly Flow was better at generics, TypeScript has some issues:

• This one prevents is required for inferring custom methods arguments: Proposal: Variadic Kinds -- Give specific types to variadic functions microsoft/TypeScript#5453

• and this one for inferring custom methods/properties return type:
  Supporting generic type inference over the other higher-order functions microsoft/TypeScript#9366

Without typechecking for arguments and return type it looks like:

interface CustomPropertiesMap {
    {[prop: string]: (node: Element) => any})
}

interface CustomPropertiesOnSnapshot<T> {
    [P in keyof T]: any
}

interface CustomPropertiesOnSelector<T> {
    [P in keyof T]: Promise<any>
}

interface TestController {
    //...
   addCustomDOMProperties<T extends CustomPropertiesMap>(props: T): CustomSelector<T>;
   //...
}

interface CustomSelector<T> extends Selector, CustomPropertiesOnSelector<T> {
    (...args: any[]): CustomSelectorPromise<T>;
}

interface CustomSelectorPromise<T> extends SelectorPromise, CustomPropertiesOnSelector<T>, Promise<NodeSnapshot & CustomPropertiesOnSnapshot<T>> {
}

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