How to test if function is called with async keyword

Question

I want to write a simple test for my vue3 app, test should assert that specific function (updateRoute in this case) is declared with async in different components

Note: according to my current project I can't isolate this function in a single file to make it reusable

example:

const updateRoute = () => { doSomethig() }

should fail

while

const updateRoute = async () => { await doSomethig() }

should pass

the testing library doesn't matter, it can be Jest or anything else

It's not shown what is doSomethig and how updateRoute is used. It could be possibly solved with a linter. This can be XY problem. Even if it's known that updateRoute returns a promise, it's unknown that it returns a correct promise, i.e doSomethig is awaited. Generally it's preferable to assert that a place where updateRoute is used works correctly, i.e. side effects from doSomethig are applied at correct time. — Estus Flask, Nov 18 '21 at 07:22

score 2 · Accepted Answer · answered Nov 18 '21 at 07:17

There is no reliable way and no reason to detect async function. For a function that returns a promise, it's the same as:

const updateRoute = () => {
  return doSomethig().then(() => {}) // Returns a promise of undefined
}

It should be tested that updateRoute returns a promise, with async or not. In Jest it's:

expect(updateRoute()).toEqual(expect.any(Promise));

As for mandatory await, this can be handled with ESLint require-await rule if necessary.

score 1 · Answer 2 · answered Nov 18 '21 at 07:11

Check if the contructor.name of the function is equal to 'AsyncFunction':

const x = async () => {};
const y = () => {}

console.log(x.constructor.name);
console.log(x.constructor.name === 'AsyncFunction');

console.log(y.constructor.name);
console.log(y.constructor.name === 'AsyncFunction');

How to test if function is called with async keyword

2 Answers2