TypeScript 2.9 adds support for number and symbol named properties in index types and mapped types.
Previously, the keyof operator and mapped types only supported string named properties.
Changes include:
An index type keyof T for some type T is a subtype of string | number | symbol.
A mapped type { [P in K]: XXX } permits any K assignable to string | number | symbol.
In a for…in statement for an object of a generic type T, the inferred type of the iteration variable was previously keyof T but is now Extract. (In other words, the subset of keyof T that includes only string-like values.)
Given an object type X, keyof X is resolved as follows:
If X contains a string index signature, keyof X is a union of string, number, and the literal types representing symbol-like properties, otherwise
If X contains a numeric index signature, keyof X is a union of
