I’m trying to understand what’s wrong in this code and why TypeScript doesn’t narrow type for ‘value’ argument.

function foo<T>(data: T, cb: (key: keyof T, value: T[typeof key]) => void): void {}

foo(
  {
    a: true,
    b: 'abc',
  },
  (key, value) => {
    switch (key) {
      case 'a':
        key; // <= "a"
        value; // <= string | boolean
               // but I expect boolean here
    }
  },
);

You can see that type guard for ‘key’ variable works perfectly but ‘value’ still remains non-narrowed. Can I make ‘value’ type based on ‘key’ type?

For example:

if (key === 'b') {
  value; // <= string
}

It looks like simple logic and nothing complex. Is it possible with current TypeScript?

TS version 5.6.2.

I have tried many combinations of generics and their inheritance. Nothing works and looks like TypeScript just can’t do this…?