Limitations

Some TypeScript types have no F# equivalent. Glutinum generates obj for them rather than failing, and this page lists what they are and what to do.

What becomes obj

Conditional types the arguments do not decide

T extends string ? A : B is resolved when T is known, when its default decides it, or when both branches are the same type. A conditional whose result depends on a type parameter of the caller is obj.

Mapped types over a type parameter

Partial<T>, Omit<T, K>, Pick<T, K> and Record<K, V> are expanded when their argument is a known type. Applied to a type parameter they are obj, and so is a recursive mapped type such as DeepPartial<T>.

Indexed access on a type parameter

T[K] and keyof T are obj unless T is known.

any, unknown, symbol, never

All four are obj.

What is dropped

Constraints on type parameters

<T extends Node> is generated as <'T>. F# has no way to express the constraint on an interface.

Partially generic overloads

A method with a defaulted type parameter, get<T = Element>(selector: string): T, gets a non-generic overload with the default applied. A method whose type parameters are only partly defaulted gets no such overload, because F# cannot choose between two generic overloads when an optional argument is omitted.

this parameters

function f(this: Window) is generated without the parameter, JavaScript does not pass it.

Escape hatches

unbox gives a value the type you know it has:

let days: ResizeArray<Date> = unbox (DateFns.eachDayOfInterval interval)

!! does the same in an expression, and createObj builds an object a ParamObject class cannot:

let feature = createObj [ "type" ==> "Feature"; "properties" ==> createObj [] ]

When an API is awkward from F#, extend the binding with a hand-written helper rather than a cast at every call site.

Report what is wrong

A construct that is generated wrongly, rather than as obj, is a bug. Reduce it to a few lines in the web application and open an issue from there.

An obj that this page does not explain is worth reporting the same way. It may be a type Glutinum can generate.