What's the cleanest way to tell typescript that each element of Object.keys(foo) is really a key of foo?

Question

playground

const foo = {
  a: 1,
  b: 2,
  c: 3
};

Object.keys(foo).forEach(key => {
  foo[key]++; // error: expression of type 'string' can't be used to index ....
})

There must be multiple ways to solve it. I'm looking for a recommended way with least impact on readability. Thanks!

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Edit:

Bonus: would be great if I can still have autocompletion when I type foo. in my codebase.

Answer 1

This should work:

(Object.keys(foo) as (keyof typeof foo)[]).forEach(key => {
    foo[key]++;
})

This isn't automatically done for you, because in theory it could be unsafe, but I played with various scenarios and couldn't produce a working demo with a runtime error.