I’m trying to do something like the following (albeit significantly more complicated):

interface Base {
    id: string;
    info: object;
}

interface A extends Base {
    example: boolean;
}

interface B extends Base {
    max: number;
}


interface ActionDataTypes {
    'for_a': A;
    'for_b': B;
}

function tryAction<A extends keyof ActionDataTypes>(id: string, action: A, data: ActionDataTypes[A]): boolean {

    switch(action){
        case 'for_a':
            //do stuff
            break;
        case 'for_b':
            //do stuff
            //this is a more complicated if statement
            if(data.max > 12){
                //do stuff
            }
            break;
    }

}

TypeScript gives the error Property 'max' does not exist on type 'A | B'. Property 'max' does not exist on type 'A'.

Any thoughts how how to fix it?

These questions did not resolve the issue:

• Typescript type safety in switch case statements
• Typescript does not consider the 'case' of a switch statement as a typeguard?
• Typescript is there a way to cast type for case in switch?
• TypeScript: Type guard does not work with switch statement when putting object into variable
• Why Typescript throw error message: Property does not exist on type?

>Solution :

Currently, the compiler is unable to type-narrow independent arguments, thus I suggest switching to a union of the args using the following mapped type:

// [action: "for_a", data: A] | [action: "for_b", data: B]
type Args = {
  [K in keyof ActionDataTypes]: [action: K, data: ActionDataTypes[K]];
}[keyof ActionDataTypes];

And redefined the tryAction as follows:

function tryAction(id: string, ...args: Args) {}

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