Typescript: How to reference property name in an string index signature definition in an interface

Illustration

export interface IEventDef {
    ready: any,
}

export interface IIPCMessage {
    eventType: keyof IEventDef,
    data: IEventDef[this['eventType']]
}

interface IMessageHandlers {
    [eventName: keyof IEventDef]: (data: ) => void
}

Question

As per the illustration giving a string index signature for an interface! As below:

interface IMessageHandlers {
    [eventName: keyof IEventDef]: (data: /*HOW_TO_REF_eventName*/) => void
}

How can we reference the eventName in my example? Or the index property name! Of the same line

In my exact code I wanted to do this:

interface IMessageHandlers {
    [eventName: keyof IEventDef]: (data: IEventDef[thisLineProp]) => void
}

Which would give a great affinity! any way?

To clear better I'll take this snippet below!

export interface IEventDef {
    ready: any,
} 

export interface IIPCMessage {
    eventType: keyof IEventDef,
    data: IEventDef[this['eventType']] // <<= here
}

In that example I was able to reference the same interface with this keyword! Which allow great type affinity! And it allowed nice typing precision!

What about referencing and getting the property name of the same line from the index signature?

Update

I noticed that I just met the

An index signature parameter type must be either 'string' or 'number'.ts(1023)`

Error!

And I changed the code to

type IMessageHandlers = {
    [eventName in keyof IEventDef]: (data: /*REF_THE_PROP_NAME?*/) => void
}

And I go with the question for this?

You are almost there actually. eventName holds the type you want.

type IMessageHandlers = {
    [eventName in keyof IEventDef]: (data: eventName) => void
}

Playground Link