Understanding async/await on NodeJS

Async functions in Javascript

The async keyword turns a regular JS function declaration into an asynchronous function declaration:

function syncFunc {// dostuff}
async function asyncFunc {// dostuff} // the async keyword is placed before the function keyword

An async function returns a Promise:

• When the async function returns a value, the Promise will be resolved with the returned value.
• When the async function throws an exception or some value, the Promise will be rejected with the thrown value.

Inside an async function you can use the await keyword. await placed before a promise causes the async function to pause until the promise is settled (either rejected or fulfilled)

• . When the promise fullfills The value of the await expression is the value of the fullfilled promise.
• When the promise is rejected the await expression throws the rejected value.

Example:

function makePromise(x) { 
    return new Promise(resolve => {
      setTimeout(() => {
        resolve(x);
      }, 1000);
    });
  }
  
  async function asyncFunc() {
    var x = await makePromise(1); // the function is paused here until the promise is fulfilled
    console.log(x); // logs 1
    return x;
  }
  
  const returnedProm = asyncFunc(); // the async func returns a promise


  returnedProm.then((x) => console.log(x));
  // This promise is fulfilled with the return value from the async func, so this logs 1

When to use asynchronous functions:

Async functions are a useful tool when you have multiple asynchronous actions (implemented as promises) which depend on each other. For example when your second promise needs data that your first promise will provide. You now can conveniently use the await keyword to first receive the data in promise 1, and then pass this data as an argument to promise 2.

In other words, async function can via the await keyword make asynchronous programming behave like synchronous programming. As a consequence your applications are easier to understand.

You can await any promise in an async function. The code after the await will be executed after the promise that you are awaiting finished.

This is a great alternative to classic JavaScript callbacks.

I wrote a blog about it -> https://github.com/Gameye/js-async I hope this will help you!

To clear a few doubts -

1. You can use await with any function which returns a promise. The function you're awaiting doesn't need to be async necessarily.
2. You should use async functions when you want to use the await keyword inside that function. If you're not gonna be using the await keyword inside a function then you don't need to make that function async.
3. async functions by default return a promise. That is the reason that you're able to await async functions.

From MDN -

When an async function is called, it returns a Promise.

As far as your code is concerned, it could be written like this -

const getUsers = (ms) => { // No need to make this async
    return new Promise(resolve => setTimeout(resolve, ms));
};

// this function is async as we need to use await inside it
export const index = async (req, res) => {
    await getUsers(5000);

    res.json([
      {
        id: 1,
        name: 'John Doe',
      },
      { id: 2,
        name: 'Jane Doe',
      },
    ]);
};