It appears that typescript permits the casting of any type to a map with keys of type number. Consider this example code:
type NumberMap = {
[key: number]: string
}
type AnyType = {
foo: string
bar: boolean[]
}
const anyTypeObj: AnyType = { foo: "foo", bar: [false] }
const numberMap: NumberMap = anyTypeObj
I initially thought that the last line would result in a type error when trying to assign AnyType to NumberMap, but surprisingly it does not. However, directly assigning an object literal to a variable of type NumberMap does lead to a type error:
const numberMap: NumberMap = { foo: "foo", bar: [false] }
View the link to see the above code on the typescript playground.
What leads typescript to allow the casting of objects of any type to NumberMap?
Is there something distinctive about the type { [key: number]: string } that causes this behavior?