Index which's key's type is number can accept string keys

When I declare an index like this:
let index: {[index: number]: string} = {};
Then I can add a member to the index like this:
index['3s'] = 3;
The type of the key '3s' is string. But this works. Is this a bug?

JavaScript objects coerce numeric indices to strings behind the scenes, leading to some modeling challenges for TypeScript. See some of the related issues for further explanation: #17191, #13715, #12425.