Ways to determine programmatically whether a function is asynchronous

Question

Ways to determine programmatically whether a function is asynchronous

My goal is to override (monkey patch) the it() function of the Jasmine framework and determine if a function passed as the second argument is of type async. I attempted using instanceof Promise since async functions return a promise, but it never evaluates to true. After inspecting logged functions, I noticed that async() function specs have a return type of tslib_1.awaiter(some args..).

Here is my current approach:

let newIt = jasmine.getEnv().it;
jasmine.getEnv().it = function(...args): jasmine.Spec {
  // perform actions.
 if(args[1] instanceOf Promise) {
   debugger; // this block is never executed.
   // handle error.
 }
 return newIt.apply(this, arguments);
}

Can you point out what I might be doing incorrectly here? Any guidance would be greatly appreciated.

Thank you.

Edit: Let's consider two dummy specs, one asynchronous and the other synchronous:

Async Test:

const exp = () => {
            return new Promise((resolve, reject) => {
                setTimeout(() => {
                    resolve(1);
                }, 500);
            });
        };

 it('asyncTest', async () => {
      expect(await exp()).toEqual(1);
  }, 600);

Synchronous:

it('testNon', () => {
     expect(true).toBe(true);
 });

javascript typescript jasmine

Answer 1

Answer №1

Here lies the issue known as the XY problem. As mentioned in this particular response, it is typically unnecessary to verify if a function is asynchronous because this distinction holds no significance. An async function simply denotes a function that returns a promise. Hence, both () => Promise.resolve() and async () => {} should be handled similarly.

This statement is further reinforced by the fact that there exists no discernible dissimilarity between an async function and a regular function within transpiled TypeScript code - they are essentially equivalent as regular functions.

The assertion args[1] instanceOf Promise is flawed since a function cannot be deemed an instance of a Promise. Typically, monkey-patching is executed in a universal manner (not limited to Jasmine or this specific scenario) through the following approach:

let newIt = jasmine.getEnv().it;
jasmine.getEnv().it = function(...args): jasmine.Spec {
  const fn = args[1];
  let newFn;
  if (fn.length > 0) {
    // async spec with done param
    newFn = function (done) {
      const result = fn.call(this, done);
      if(result instanceOf Promise)
      // Alternatively,
      // if(result && typeof result.then === 'function')
        debugger;
      return result;
    }
  } else {
    newFn = function () {
      const result = fn.call(this);
      if(result instanceOf Promise)
        debugger;          
      return result;
    }
  }

  args[1] = newFn;
  return newIt.apply(this, args);;
}

What demands scrutiny is the outcome of the test function result, which should be examined to ascertain if it aligns with being an instance of a Promise (or if it's deemed thenable).

Answer 2

Here lies the issue known as the XY problem. As mentioned in this particular response, it is typically unnecessary to verify if a function is asynchronous because this distinction holds no significance. An async function simply denotes a function that returns a promise. Hence, both () => Promise.resolve() and async () => {} should be handled similarly.

This statement is further reinforced by the fact that there exists no discernible dissimilarity between an async function and a regular function within transpiled TypeScript code - they are essentially equivalent as regular functions.

The assertion args[1] instanceOf Promise is flawed since a function cannot be deemed an instance of a Promise. Typically, monkey-patching is executed in a universal manner (not limited to Jasmine or this specific scenario) through the following approach:

let newIt = jasmine.getEnv().it;
jasmine.getEnv().it = function(...args): jasmine.Spec {
  const fn = args[1];
  let newFn;
  if (fn.length > 0) {
    // async spec with done param
    newFn = function (done) {
      const result = fn.call(this, done);
      if(result instanceOf Promise)
      // Alternatively,
      // if(result && typeof result.then === 'function')
        debugger;
      return result;
    }
  } else {
    newFn = function () {
      const result = fn.call(this);
      if(result instanceOf Promise)
        debugger;          
      return result;
    }
  }

  args[1] = newFn;
  return newIt.apply(this, args);;
}

What demands scrutiny is the outcome of the test function result, which should be examined to ascertain if it aligns with being an instance of a Promise (or if it's deemed thenable).

Answer 3

Answer №2

The reason for the failure lies in the fact that async functions always yield a promise, yet they do not inherently qualify as promises themselves.

A straightforward approach would involve inspecting the function's constructor (although I will elaborate on why this may not be advisable):

const isAsync = func.constructor.name === 'AsyncFunction';

The flaw with this method is that async functions are typically indistinguishable from regular functions that return promises. It would be more prudent to utilize Promise.resolve to convert any function call output into a Promise, which can then be awaited.

Answer 4

The reason for the failure lies in the fact that async functions always yield a promise, yet they do not inherently qualify as promises themselves.

A straightforward approach would involve inspecting the function's constructor (although I will elaborate on why this may not be advisable):

const isAsync = func.constructor.name === 'AsyncFunction';

The flaw with this method is that async functions are typically indistinguishable from regular functions that return promises. It would be more prudent to utilize Promise.resolve to convert any function call output into a Promise, which can then be awaited.

Ways to determine programmatically whether a function is asynchronous

Answer №1

Answer №2

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