In this code snippet, we have a function that accepts a promise and a timeout value as arguments, then returns another promise. If the initial promise does not resolve within the specified timeout period, the new promise will reject immediately. On the other hand, if the initial promise resolves before the timeout, the new promise will also resolve with the same value.
function resolveOrTimeout<T>(promise: Promise<T>, timeout: number): Promise<T> {
return new Promise<T>((resolve, reject) => {
// set up timeout functionality
const task = setTimeout(() => reject("time up!"), timeout);
promise.then(val => {
// cancel the timeout
clearTimeout(task);
// resolve with the value from the original promise
resolve(val);
});
});
}
resolveOrTimeout(fetch(""), 3000);
The concept behind this function is clear, however, I am puzzled by the type annotations used here, particularly the implementation of generics. Generics in TypeScript are used to parameterize types, similar to how functions parameterize values. But in this case, why do we need to use generics for types? Moreover, even when the type variables like T are not explicitly provided during the function call, the compiler does not flag any errors. This behavior seems perplexing to me.