async-await.md

https://javascript.info/async-await

The async keyword before a function has two effects:

• Makes it always return a promise.
• Allows await to be used in it.

The await keyword before a promise makes JavaScript wait until that promise settles, and then:

• If it’s an error, the exception is generated — same as if throw error were called at that very place.
• Otherwise, it returns the result.

The following are same

async function f() {
  await Promise.reject(new Error("Whoops!"));
}

async function f() {
  throw new Error("Whoops!");
}

We can catch that error using try..catch, the same way as a regular throw:

async function f() {

  try {
    let response = await fetch('http://no-such-url');
  } catch(err) {
    alert(err); // TypeError: failed to fetch
  }
}

f();

If we don’t have try..catch, then the promise generated by the call of the async function f() becomes rejected. We can append .catch to handle it:

async function f() {
  let response = await fetch('http://no-such-url');
}

// f() becomes a rejected promise
f().catch(alert); // TypeError: failed to fetch // (*)

If we forget to add .catch there, then we get an unhandled promise error (viewable in the console). We can catch such errors using a global unhandledrejection event handler as described in the chapter Error handling with promises.

An example for error handling in mongoose

mongoose.connect('mongodb://127.0.0.1:27017/test').
  catch(error => handleError(error));

// Or:
try {
  await mongoose.connect('mongodb://127.0.0.1:27017/test');
} catch (error) {
  handleError(error);
}