Async function as callback

Question

I just started using async/await and is confused on how it interacts with callback. For example,

fooMethod(function() {
    return Promise.resolve("foo");
});

vs

fooMethod(async function() { //add async keyword
    return "foo";
});

Must fooMethod be written in a specific way so that it can handle an async function as callback?

if fooMethod is a public library, how do I know that it is safe to add async keyword to the function?

FOLLOW UP

Express router,

app.get('/foo', function (req, res) {
  return res.send("foo");
});



app.get('/foo', async function (req, res) {
  return res.send("foo");
});

both of these function works, is it safe to use though?

Answer 1

Your two callbacks are equivalent. An async function is just syntactic sugar for a regular function that returns a Promise. This means that you can call an async function like a regular function. Here’s a demo:

const foo = async function (arg) {
  return arg * 2
}
const bar = function (arg) {
  return Promise.resolve().then(() => {
    return arg * 2
  })
}

const fooReturn = foo(2)
const barReturn = bar(2)
console.log('foo(2) =>', fooReturn.toString())
console.log('bar(2) =>', barReturn.toString())

fooReturn.then(fooResult => console.log('await foo(2) =>', fooResult))
barReturn.then(barResult => console.log('await bar(2) =>', barResult))

However, if the code that takes the callback wants to get a response, you won’t be able to use an async function unless the code is specially designed to check the return value of the callback function and await it if it’s a Promise.