const palette: { [key: string]: string } = {...

Object literals don't have index signatures. They are assignable to types with index signatures if they have compatible properties and are fresh (i.e. provably do not have properties we don't know about) but never have index signatures implicitly or explicitly.

Which... is confusing because Palette technically does have an index signature Palette is a mapped type, and mapped types don't have index signatures. The fact that both use [ ] is a syntactic coincidence.

Generate type with the index signature: interface RandomMappingWithIndexSignature { stringProp: string; numberProp: number; [propName: string]: string | number | undefined; }

we have no way to know that the line nameMap[3] = "bob"; isn't somewhere in your program

The other commenters are right about the potential solutions. However, it is actually considered a best practice to move the object with the index signature to a nested property.Said differently: No property in the object with the index signature should depart from how the index signature is typed.

Like others have noted, your function does not conform to index signature. [key: string]: string means "all fields are strings" and on the next line you declare a field with function in it.This can be solved by using union types:type IFoo = { [foo: string]: string } & { fooMethod(fooParam: string): void }

So now the question is, why does Session, an interface, not get implicit index signatures while SessionType, an identically-structured typealias, *does*? Surely, you might again think, the compiler does not simply deny implicit index signatures tointerface` types? Surprisingly enough, this is exactly what happens. See microsoft/TypeScript#15300, specifically this comment: Just to fill people in, this behavior is currently by design. Because interfaces can be augmented by additional declarations but type aliases can't, it's "safer" (heavy quotes on that one) to infer an implicit index signature for type aliases than for interfaces. But we'll consider doing it for interfaces as well if that seems to make sense And there you go. You cannot use a Session in place of a WithAdditionalParams<Session> because it's possible that someone might merge properties that conflict with the index signature at some later date. Whether or not that is a compelling reason is up for rather vigorous debate, as you can see if you read through microsoft/TypeScript#15300.

why is Session not assignable to WithAdditionalParams<Session>? Well, the type WithAdditionalParams<Session> is a subtype of Session with includes a string index signature whose properties are of type unknown. (This is what Record<string, unknown> means.) Since Session does not have an index signature, the compiler does not consider WithAdditionalParams<Session> assignable to Session.