using async await and .then together

I don't think mixed use async/await + then is a good idea. Especially when you focus on the async func's res, mixed use will bring some risk to distort your res silently.

example:

const res = await fetchData().then(()=>{return "something else"}).catch((err)=>{});

console.log(res); // not expected res data but something else

So, mixed use is dangerous , by the way, it'll do harm to read codes.

I always use async/await and .catch() instead of using async/await and try/catch to make code compactly.

async function asyncTask() {
  throw new Error('network')
}
async function main() {
  const result = await asyncTask().catch(error => console.error(error));
  console.log('result:', result)
}

main();

If you want to get a fallback value when an error happened, you can ignore the error and return a value inside the .catch() method

async function asyncTask() {
  throw new Error('network')
}
async function main() {
  const result = await asyncTask().catch(_ => 'fallback value');
  console.log('result:', result)
}

main();

An async function can contain an await expression that pauses the execution of the async function and waits for the passed Promise's resolution, and then resumes the async function's execution and returns the resolved value.

As you can see from below example that you can use two ways to handle await result and errors,The keyword await makes JavaScript wait until that promise settles and returns its result (One you get from resolved promise).So as such there is no harm (I don't fully understand what you refer as harm here).

function returnpromise(val) {
  return new Promise((resolve, reject) => {
    if (val > 5) {
      resolve("resolved"); // fulfilled
    } else {
      reject("rejected"); // rejected
    }
  });
}

//This is how you handle errors in await
async function apicall() {
  try {
    console.log(await returnpromise(5))
  } catch (error) {
    console.log(error)
  }
}

async function apicall2() {
  let data = await returnpromise(2).catch((error) => {
    console.log(error)
  })
}

apicall2();
apicall();

For further reference have a look at-MDN DOCS

Don't want to raise the dead, but want to point out that using await along with a then chain means that the result of:

const x = await someAsyncFn().then(() => doSomeLogging());

The value of x is assigned the return value of .then (i.e. undefined, if doSomeLogging is void) which wasn't super intuitive to me.