Is it possible to define an annotation for a function that accepts a callback, and have the function return type determined by the callback's return type?
// Suppose the callback takes a number as argument
function processCallback(cb: (arg:number) => infer T /* error */ ) {
return cb(42)
}
processCallback(x => 'bar') // expected to infer 'string'
If you're looking to implement the ReturnType helper, don't forget to adjust the generic structure so it can be restricted to a function.
function executeCallback<T extends (...args: unknown[]) => unknown>(callback: T): ReturnType<T> {
return callback(42) as ReturnType<T>;
}
const result1 = executeCallback(x => 'foo'); // string
const result2 = executeCallback(x => 123); // number
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