tsc doesn't recognize coercion of object keys to strings

[cmwilhelm]

I ran into a scenario yesterday that surprised me a bit. I was using an object literal to store some unique keys that I later extracted with a for-in loop. The keys were all numbers, but I quickly ran into a problem at runtime because i had forgotten that JavaScript coerces number type keys to strings for objects.

To wit, given the coercion of keys to strings, I was surprised that this type checked:

var ids: Array<number> = [];
const myCollection = {
    0: true,
    1: true
};

for (var key in myCollection) {
    ids.push(key);
}

console.log(ids); // => ["0", "1"]

Is this expected behavior, or should this throw a compile time warning?

(TSC Version 1.7.5)

[vladima]

duplicate of #4518