Typescript mapped interfaces

Question

I have this interface:

interface Events {
  SERVER_START: {
    version: string;
  };
  SERVER_READY: {
    host: string;
    port: number;
  };
  SERVER_STOP: undefined;
  SERVER_STOP_ERROR: {
    error: Error;
  };
  SERVER_CONNECTION_INCOMING: {
    clientId: string;
    address: string;
    port: number;
  };
 ...
}

is it possible to map functions parameters like this?

function logEvent(type: keyof Events, data?: Events[keyof Events]): void;

This work almost fine, since I can call logEvent like this:

// this is correct:
logEvent('SERVER_START', { version: '0.1.0' });

// but this is also correct, which shouldn't
logEvent('SERVER_START');
logEvent('SERVER_START', { host, port });

Of course, intellisense also don't work as expected so, I had to manually define all the overloadings of the function like...

function logEvent(type: 'SERVER_START', data: Events['SERVER_START']): void;
function logEvent(type: 'SERVER_READY', data: Events['SERVER_READY']): void;
function logEvent(type: 'SERVER_STOP'): void;
...

which works like a charm, but it looks to me like there should be a better way to define all of this automagically...

Update:

So, the idea is not having only Events but extending that interface with other definitions as well, so the idea would be to have a generic function like

function logEvent<E extends Events>(type: keyof E, data?: E[keyof E]): void;

so you can actually do

interface MoreEvents extends Events {
  FOOBAR: {
    foo: string;
    bar: number;
  };
}

// call it with the original ones
logEvent('SERVER_START', { version: '0.1.0' });

// call it also with the new events
logEvent<MoreEvents>('FOOBAR', { foo, bar });

but with proper type checking

ThomasThiebaud · Accepted Answer · 2019-10-14 14:45:02Z

1

You can redefine your logEvent function using generics

logEvent<E extends Events>(type: keyof E, data?: E[keyof E]): void;

Update

This solution works with the updated question. Check that playground

edited Oct 14, 2019 at 14:45

answered Oct 14, 2019 at 12:09

ThomasThiebaud

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5 Comments

danikaze Over a year ago

omg so stupid solution... I tried lots of them but didn't get the correct syntax. Thanks this works. By the way, making the second parameter optional works just fine too :)

ThomasThiebaud Over a year ago

Happy to help. I was not sure about the second argument

danikaze Over a year ago

wait, if T extends keyof Events that means that, there could be some values which will Events[T] will be undefined, right? So that's not really valid...

ThomasThiebaud Over a year ago

To answer your comment if you have logEvent<T extends keyof Events> then T will always be one of the key of Events, so no undefined

ThomasThiebaud Over a year ago

It also works with the updated question, check the playground in the updated answer

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