Automatic type inference for functions in TypeScript with arguments

I am looking to define an interface with the following structure:

interface CheckNActSetup<D, C> {
defs: (event: Event) => D,
context: (defs: D) => C;
exec: (context: C) => any[];
when: ((context: C) => boolean)[]; }

and implement it using a class method:

class Test {
    register<C>(item: CheckNActSetup<C>) {

    }
}

When I attempt to use the method as shown below:

let x = new Test();
x.register({
   context: e => ({ value: "A" }),
   exec: context => context.value
})

I encounter an issue where the 'context' parameter in the 'exec' property is unknown. My goal is for the compiler to infer that the context type is: { value: string }

To achieve this without adding (e: Event), my question is:

How can I determine the proper context variable type without explicitly specifying (e: Event)?</p>

Answer №1

I tried running your code on TypeScript 4.7 and it worked perfectly, but it doesn't compile on version 4.6 due to the error Object is of type 'unknown'.

To resolve this issue, please refer to TypeScript 4.7 Release Notes: Improved Function Inference in Objects and Methods

The latest update in TypeScript 4.7 allows for more precise inferences from functions within objects and arrays, ensuring consistent typing flow from left to right just like regular arguments.

declare function f<T>(arg: {
   produce: (n: string) => T,
   consume: (x: T) => void }
): void;
// Example snippets showcasing improved inference
// ...

In some cases, inference may have failed because determining the type of produce functions indirectly requests arg's type prior to settling on a suitable type for T. TypeScript now handles these scenarios by selectively inferring from relevant functions contributing to the inferred type of T in a lazy manner.

If you need further details, you can delve into the specific adjustments made to our inference process.

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