TypeScript wrapper function with additional parameters

Question

I want to create a wrapper function for an existing function in TypeScript.

The wrapper function could start some other process and clean it up after finishing the main ("callback") function passed to the wrapper.

This can be done using approaches like shown here. However, these solutions do not allow me to specify additional options that can be passed to the wrapper itself.

How would I go about doing that?

My starting point was:

export const wrap = async <T>(
  callback: () => T | Promise<T>,
  options?: { foo?: string | undefined },
): Promise<T> => {
  let ret;

  // begin
  if (options.foo) {
    // do something
  }

  try {
    ret = await callback();
  } catch (e) {
    throw e;
  } finally {
    // cleanup
  }

  return ret;
};

This would not let me add arguments to callback(). I can use ...args, but how would I specify both ...args and options?

Guerric P · Accepted Answer · 2022-11-15 11:47:35Z

2

I suggest you to write your function as a factory (function that returns another function):

function wrap<T extends (...args: any[]) => any>(callback: T, options?: { foo?: string | undefined }): (...args: Parameters<T>) => ReturnType<T> extends Promise<infer U> ? Promise<U> : Promise<ReturnType<T>>
function wrap(callback: (...args: any[]) => any, options?: { foo?: string | undefined }): (...args: any[]) => Promise<any> {

  return async function (...args: any[]) {
    let ret;

    // begin
    if (options && options.foo) {
      // do something
    }

    try {
      ret = await callback(...args);
    } catch (e) {
      throw e;
    } finally {
      // cleanup
    }

    return ret;
  }
};

async function asyncExtractFirstParameter(str: string, num: number, bool: boolean) {
  return str;
}

function extractFirstParameter(str: string, num: number, bool: boolean) {
  return str;
}

const wrappedAsyncExtractFirstParameter = wrap(asyncExtractFirstParameter);
const wrappedExtractFirstParameter = wrap(extractFirstParameter);

wrappedAsyncExtractFirstParameter('test', 23, true).then(console.log);
wrappedExtractFirstParameter('test', 23, true).then(console.log);

That allows you to create another function with the same signature as the callback, and possibly defer its execution.

TypeScript playground

edited Nov 15, 2022 at 11:47

answered Nov 3, 2022 at 12:50

Guerric P

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3 Comments

slhck Over a year ago

Thanks, that is nice. I don't necessarily need to defer here, but this is more generally usable.

slhck Over a year ago

Coming back to this, can I somehow type the ...args: any[] so that instead it validates the actual parameters of the function passed? I guess I could use ReturnType<T> and Parameters<T> but I am not sure how.

Guerric P Over a year ago

I have updated my answer with a possible solution

slhck · Accepted Answer · 2022-11-03 12:40:23Z

The solution consists of making the functon generic (F) and inferring the parameters and return types of that function via ReturnType and Parameters. Any additional options can be specified in advance of the destructured remaining arguments which are passed to the callback function:

export const wrap = async <F extends (...args: any[]) => ReturnType<F> | Promise<ReturnType<F>>>(
  callback: F,
  options: { foo?: string | undefined } = {},
  ...args: Parameters<F>
): Promise<ReturnType<F>> => {
  let ret: ReturnType<F> | undefined;

  // begin
  if (options.foo) {
    // do something
  }

  try {
    ret = await callback(...args);
  } catch (e) {
    throw e;
  } finally {
    // cleanup
  }

  return ret;
};

The wrapper can then be called like this:

const someFunction = (arg1: string, arg2: boolean): void => {
  // do something
}

wrap(someFunction, { foo: "yes" }, arg1, arg2)`

All return types and argument types are inferred automatically.

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