How to set function's param as a key from interface

Question

I have interface and object which represents this interface:

MyInterface {
  variableOne: string;
  variableTwo: boolean;
  variableThree: boolean; 
  ...
}

Also I have function toggleFunction(x) which gets a key and toggles it in my object. Which type do I need to set in param x in my toggle function to use it for toggle all boolean key in my object?

For example I need something like: toggleFunction(x: TYPE?)

Answer 1

Use keyof.

function toggleFunction(x: keyof MyInterface): void {
    // ...
}

Answer 2

You can use a conditional mapped type that I usually call KeysMatching:

type KeysMatching<T, V> = { [K in keyof T]: T[K] extends V ? K : never }[keyof T];

And then toggleFunction() can be defined like this:

declare const o: MyInterface; // or wherever it comes from
function toggleFunction(x: KeysMatching<MyInterface, boolean>) {
    o[x] = !o[x];
}

toggleFunction("variableTwo"); // okay
toggleFunction("variableThree"); // okay
toggleFunction("variableOne"); // error!

Hope that helps; good luck!

Link to code