The KeyValuePair<string, Date> type in Typescript cannot be assigned to the KeyValuePair<number, string> type

I encountered the following issue:

An error occurred stating that Type 'KeyValuePair<string, Date>' is not assignable to type 'KeyValuePair<number, string>'. 
  Also, it mentioned that Type 'string' is not assignable to type 'number'.

This problem is related to the code snippet below:

class KeyValuePair<TKey, TValue> {
    constructor(public key: TKey, public value: TValue) {}
}

class KeyValuePairPrinter<T, U> {
    constructor(private pairs: KeyValuePair<T, U>[]) {}

    print() {
        for (let p of this.pairs) {
            console.log(`${p.key}: ${p.value}`);
        }
    }
}

let now = new Date(Date.now());
let pair1 = new KeyValuePair<number, string>(1, "First");
let pair2 = new KeyValuePair<string, Date>("Second", new Date(Date.now()));
let pair3 = new KeyValuePair<number, string>(3, "Third");

var printer = new KeyValuePairPrinter([
    pair1,
    pair2,
    pair3,
]);
printer.print();

You can view the screenshot for reference here: https://i.stack.imgur.com/Md2LQ.png

What do you think might be causing this error?

Answer №1

For your specific scenario, TypeScript automatically determines the type of the generic argument based on the initial value passed to the constructor.

let item1 = new ItemPair<number, string>(1, "First");
let item2 = new ItemPair<string, Date>("Second", new Date(Date.now()));
let item3 = new ItemPair<number, string>(3, "Third");

var printer = new ItemPairPrinter([
    item1,
    item2, 
    item3
]);

To override TypeScript's inferred type, you can explicitly define the generic argument types as follows:

var printer = new ItemPairPrinter<number | string, string | Date>([
    item1, 
    item2,
    item3,
]);

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