CODEWITHSUNDEEP
typescript
mikeysee
mikeysee

Reputation: 1763

How to define a callback as one of two types?

Im trying to define some types that say that a callback must be used in one of two ways, either it must be an error in which case it should not have a result, or it must be a result in which case it must not have an error.

So as an example here is a method where im trying to use it:

import { LambdaCallback } from "./types";

export function handler(event:any, context:any, callback:LambdaCallback) {
    console.log(event)
    callback(null, {
      statusCode: 200,
      body: JSON.stringify({msg: "Hello, World!!"})
    })
  }

This is my attempt at the types:

export type LambdaCallbackError = string | {};
export type LambdaCallResult = {
    statusCode: number,
    body: string
}

export type LambdaCallbackWithError = (error:LambdaCallbackError) => void;
export type LambdaCallbackWithResult = (error:null, result:LambdaCallResult) => void;
export type LambdaCallback = LambdaCallbackWithError | LambdaCallbackWithResult;

The types compile fine but im getting the following error when trying to use it:

lambda/hello.ts(6,5): error TS2349: Cannot invoke an expression whose type lacks a call signature. Type 'LambdaCallback' has no compatible call signatures.

Any ideas?

Upvotes: 2

Views: 142

Answers (1)

basarat
basarat

Reputation: 276085

Your code LambdaCallbackWithError | LambdaCallbackWithResult

This is wrong. You do not have a single function that can be one of those. You have a single function that is both of those.

You should: 

export type LambdaCallback = (error:null | LambdaCallbackError, result:LambdaCallResult) => void;

Upvotes: 4

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