How to define an argument where only one property needs to match?

[steep shell]

This example complains for the second method call because the arg I'm passing also has a b property. How can I define the type for the function argument so that it accepts either method call?

[pastel geyser]

claudekennilol#0

Playground Link

Preview:```ts
const foo = (arg: {f: string}) => {}

foo({f: ""})
foo({f: "", b: ""})```

You can choose specific lines to embed by selecting them before copying the link.

[distant silo]

There is nothing wrong with the function. It is TS Excess Property Checks

[pastel geyser]

sandiford#0

Playground Link

Preview:```ts
const foo = (arg: {f: string}) => {}

const one = {f: ""}
const two = {f: "", b: ""}

foo(one)
foo(two)```

You can choose specific lines to embed by selecting them before copying the link.

[distant silo]

all works

!hb excess

!hb excess property checks

https://www.typescriptlang.org/docs/handbook/2/objects.html#excess-property-checks

[steep shell]

What do you mean "all works"?

Do you just mean "any of the following listed here"?

[distant silo]

Check my playground link

I mean that your object types work with your function.

[steep shell]

Why is it fine (for typescript) to pass a reference to the function, but it complains if the object is passed directly to the function?

[distant silo]

TypeScript applies excess property checks when you create an object of a specific type

and when you are creating the object as an argument, it treats the argument type as the type of object you are creating

[pastel geyser]

const o: { a: string } = { a: 'foo', b: 'bar' } as { a: string }
//    ^? - const o: {
//        a: string;
//    }
 o.b
// ^
// Property 'b' does not exist on type '{ a: string; }'.
// ^? - any

[distant silo]

Because you can never access that b property

It's not technically illegal according to the type system, but TS treats it as an error, to catch likely mistakes

It would be easy to mistype an optional property and not get the object you wanted otherwise

[steep shell]

Fair, but I'd expect a different error if it's missing a property that the calling function explicitly expects. I knew that it was valid js (excluding the inline typing in the function arg declaration) but I wasn't sure exactly what ts's problem with it was.

!solved

[distant silo]

That's why I said an optional property

There is no error for missing an optional

[pastel geyser]

const foo = (arg: { One: string, Two?: string }) => {}

               // incorrect casing
foo({ One: "", two: "" });
//             ^^^
// Object literal may only specify known properties, but 'two' does not exist in type '{ One: string; Two?: string | undefined; }'. Did you mean to write 'Two'?