Conditional parameters of types based on strings in TypeScript

Question

I would like to have a method that takes action as first argument, and payload is decided based upon the the first argument.

Let's say I have a method as below:

  async executeUpdateAction<K>(
    action_type: K,
    action_payload: TransferUpdateActions[K],
  ) {
    if (action_type === TRANSFER_AVAILABLE_UPDATE_ACTIONS.CANCEL) {
      return this.cancelTransfer(payload);
    }

    if (action_type === TRANSFER_AVAILABLE_UPDATE_ACTIONS.HOLD) {
      return this.holdTransfer(payload);
    }
   ...
}

This is simply an abstraction that invokes a function based on action type.

Each underlying method (i.e. cancelTransfer, holdTransfer) has its own unique payload requirement.

For instance, lets say holdTransfer requires the following parameters:

  interface HoldPayload {
    duration: string; 
  }

  holdTransfer(payload: HoldPayload) {
    ... 
  }

This being the case, for method executeUpdateAction, if the first argument, action_type is equal to "hold", I want type auto completion so that payload is typed as HoldPayload.

Lastly, I have a wrapper that invokes executeUpdateAction, as below:

interface UpdateDto {
   duration?: string, // payload argument required to execute "Hold" 
   admin_id?: number // payload argument required to execute "Cancel" 
}

update(payload: UpdateDTO) {
   this.executeUpdateAction(payload)
}

So as long as UpdateDTO payload matches ONE of Cancel or Hold Payload, it should be type-safe.

Please advise.

Make sure you look at the tag's details. Typescript-typings is a very specific thing that is not related to your question at all. — crashmstr
– crashmstr, Commented Sep 15, 2021 at 16:38

aelesia · Accepted Answer · 2021-09-15 17:04:40Z

1

Try this:

type Action = "cancel" | "hold";
type Payload = {
  cancel: {
    userId: string;
  };
  hold: {
    userId: string;
    duration: number;
    amount: number;
  };
};

function executeAction<T extends Action>(action: T, payload: Payload[T]) {
  return;
}

Seems like Typescript doesn't seem to be able to narrow down generics for the second param. In this case you can safely cast your payload.

function executeAction<T extends Action>(action: T, payload: Payload[T]) {
  if (action === "cancel") {
    cancelTransfer(payload as Payload["cancel"]);
  }
}

edited Sep 15, 2021 at 17:04

answered Sep 15, 2021 at 16:33

aelesia

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2 Comments

paul.kim1901 Over a year ago

if (action_type === TRANSFER_AVAILABLE_UPDATE_ACTIONS.hold) { return this.holdTransfer(payload); } hold(payload: TransferUpdatePayload['hold']) i get following error: Argument of type '{ amount: number; } | { type: DocumentResubmissionType[]; } | { partner_id: number; } | undefined' is not assignable to parameter of type '{ amount: number; }'. Type 'undefined' is not assignable to type '{ amount: number; }'

aelesia Over a year ago

Updated my answer. You can try to cast it.

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