Can you declare a object literal type that allows unknown properties in typescript?

Question

Essentially I want to ensure that an object argument contains all of the required properties, but can contain any other properties it wants. For example:

function foo(bar: { baz: number }) : number {
    return bar.baz;
}

foo({ baz: 1, other: 2 });

But this results in:

Object literal may only specify known properties, and 'other' does not exist in type '{ baz: number; }'.

Answer 1

Yes, you can. Try this:

interface IBaz {     baz: number;     [key: string]: any; }  function foo(bar: IBaz) : number {     return bar.baz; }  foo({ baz: 1, other: 2 }); 

Answer 2

Well, i hate answering my own questions, but the other answers inspired a little thought... This works:

function foo<T extends { baz: number }>(bar: T): void {     console.log(bar.baz); }  foo({baz: 1, other: 2}); 

Answer 3

If the known fields are coming from a generic type the way to allow wildcards is with T & {[key: string]: unknown}, any fields that are known must fit with the type's constraints and other fields are allowed (and considered type unknown)

Here is a sample:

type WithWildcards<T> = T & { [key: string]: unknown };

function test(foo: WithWildcards<{baz: number}>) {}

test({ baz: 1 }); // works
test({ baz: 1, other: 4 }); // works
test({ baz: '', other: 4 }); // fails since baz isn't a number

Then if you have a generic type T you can allow wildcard fields with WithWildCards<T>

Note that extra properties are not marked as errors if the object is coming from anything other than an object literal, TS is just telling you that with the typing putting that property in the literal is extraneous.

Here are some other cases where extra properties are and aren't allowed

interface Foos{
  a?: string
  b?: string
}
type WithWildcards<T> = T & { [key: string]: unknown };

declare function acceptFoos(foo: Foos): void;
declare function acceptWild(foo: WithWildcards<Foos>):void

acceptFoos(  {a:'', other: ''}) // ERROR since the property is extraneous
const data = {a:'', other: ''}
acceptFoos(  data) // allowed since it is compatible, typescript doesn't force you to remove the properties
const notCompat = {other: ''}
acceptFoos(notCompat) //ERROR: Type '{ other: string; }' has no properties in common with type 'Foos'
acceptFoos({a:'', ...notCompat}) // this is also allowed 
acceptWild(notCompat) // allowed

Answer 4

If you want to allow unknown properties for the entire object, you can use Record

function doStuff(payload: Record<string|number, unknown>): Record<string|number, unknown> {
  return { anyProp: 'anyValue' }
}