How to define specific return type for known args?

[leaden bear]

Let's say I've got this method obj.getProperty(key) where the type for each property depends on the specific key.

How can I define the type (specifically in a .d.ts file) so that known keys return defined types, but the rest return unknown?

So basically

interface Obj {
  getProperty('foo'): Foo;
  getProperty('bar'): Bar;
  getProperty(key: string): unknown;
}

[spiral kraken]

you almost had the right syntax

@leaden bear

don't forget the parameters do need names

so that would give something like

interface Obj {
    getProperty(key: 'foo'): Foo;
    getProperty(key: 'bar'): Bar;
    getProperty(key: string): unknown;
}

this is called an overload btw, you can find those in the docs

!hb overload

[thorn pasture]

Declaration Files / Declaration Reference

Overloaded Functions

[leaden bear]

d'oh, so simple, thanks 👍

!resolved

p.s. It looks like the most-generic definition has to be first, VS Code is complaining about the "unknown" usages otherwise.

[spiral kraken]

no, the opposite

the most precise definitions have to go first

VS Code is complaining about the "unknown" usages otherwise.
are you sure your parameter is typed as "foo" and not as string when passed to the function?

[leaden bear]

That's what it gives me when I put the most-specific first

Where the right is my .d.ts file

[spiral kraken]

don't think the syntax is right

[leaden bear]

Ok nevermind, all solved now, yeah I just realized I used the completely wrong syntax on the right

[spiral kraken]

declare is usually used for 1 object at a time, not sure it works with overloads