lecture-04.md

Episode 23 : async await

Topics Covered

• What is async?
• What is await?
• How async await works behind the scenes?
• Example of using async/await
• Error Handling
• Interviews
• Async await vs Promise.then/.catch

Q: What is async?
A: Async is a keyword that is used before a function to create a async function.

Q: What is async function and how it is different from normal function?

// 💡 async function always returns a promise, even if I return a simple string from below function, async keyword will wrap it under Promise and then return.
async function getData() {
  return "Namaste JavaScript";
}
const dataPromise = getData();
console.log(dataPromise); // Promise {<fulfilled>: 'Namaste JavaScript'}

//❓How to extract data from above promise? One way is using promise .then
dataPromise.then(res => console.log(res)); // Namaste JavaScript

Another example where async function is returning a Promise

const p = new Promise((resolve, reject) => {
  resolve('Promise resolved value!!');
})

async function getData() {
  return p;
}
// In above case, since we are already returning a promise async function would simply return that instead of wrapping with a new Promise.
const dataPromise = getData();
console.log(dataPromise); // Promise {<fulfilled>: 'Promise resolved value!!'}
dataPromise.then(res => console.log(res)); // Promise resolved value!!

Q: How we can use await along with async function?
A: async and await combo is used to handle promises.

But Question is how we used to handle promises earlier and why we even need async/await?

const p = new Promise((resolve, reject) => {
  resolve('Promise resolved value!!');
})

function getData() {
  p.then(res => console.log(res));
}

getData(); // Promise resolved value!!

//📌 Till now we have been using Promise.then/.catch to handle promise.
// Now let's see how async await can help us and how it is different

// The rule is we have to use keyword await in front of promise.
async function handlePromise() {
  const val = await p;
  console.log(val);
}
handlePromise(); // Promise resolved value!!

📌 await is a keyword that can only be used inside a async function.

await function() {} // Syntax error: await is only valid under async function.

Q: What makes async-await special?
A: Let's understand with one example where we will compare async-await way of resolving promise with older .then/.catch fashion. For that we will modify our promise p.

const p = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve('Promise resolved value!!');
  }, 3000);
})

// Let's now compare with some modification:

// 📌 Promise.then/.catch way
function getData() {
  // JS engine will not wait for promise to be resolved
  p.then(res => console.log(res));
  console.log('Hello There!');
}

getData(); // First `Hello There!` would be printed and then after 3 secs 'Promise resolved value!!' will be printed.
// Above happened as Javascript wait for none, so it will register this promise and take this callback function and register separately then js will move on and execute the following console and later once promise is resolved, following console will be printed.

//❓ Problem: Normally one used to get confused that JS will wait for promise to be resolved before executing following lines.

// 📌 async-wait way:
async function handlePromise() {
  // JS Engine will waiting for promise to resolve.
  const val = await p;
  console.log('Hello There!');
  console.log(val);
}
handlePromise(); // This time `Hello There!` won't be printed immediately instead after 3 secs `Hello There!` will be printed followed by 'Promise resolved value!!'
// 💡 So basically code was waiting at `await` line to get the promise resolve before moving on to next line.

// Above is the major difference between Promise.then/.catch vs async-await

//🤓 Let's brainstorm more around async-await
async function handlePromise() {
  console.log('Hi');
  const val = await p;
  console.log('Hello There!');
  console.log(val);

  const val2 = await p;
  console.log('Hello There! 2');
  console.log(val2);
}
handlePromise(); 
// In above code example, will our program wait for 2 time or will it execute parallely.
//📌 `Hi` printed instantly -> now code will wait for 3 secs -> After 3 secs both promises will be resolved so ('Hello There!' 'Promise resolved value!!' 'Hello There! 2' 'Promise resolved value!!') will get printed immediately.

// Let's create one promise and then resolve two different promise.
const p2 = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve('Promise resolved value by p2!!');
  }, 2000);
})

async function handlePromise() {
  console.log('Hi');
  const val = await p;
  console.log('Hello There!');
  console.log(val);

  const val2 = await p2;
  console.log('Hello There! 2');
  console.log(val2);
}
handlePromise(); 
// 📌 `Hi` printed instantly -> now code will wait for 3 secs -> After 3 secs both promises will be resolved so ('Hello There!' 'Promise resolved value!!' 'Hello There! 2' 'Promise resolved value by p2!!') will get printed immediately. So even though `p2` was resolved after 2 secs it had to wait for `p` to get resolved


// Now let's reverse the order execution of promise and observe response.
async function handlePromise() {
  console.log('Hi');
  const val = await p2;
  console.log('Hello There!');
  console.log(val);

  const val2 = await p;
  console.log('Hello There! 2');
  console.log(val2);
}
handlePromise(); 
// 📌 `Hi` printed instantly -> now code will wait for 2 secs -> After 2 secs ('Hello There!' 'Promise resolved value by p2!!') will get printed and in the subsequent second i.e. after 3 secs ('Hello There! 2' 'Promise resolved value!!') will get printed

Q: Question is Is program actually waiting or what is happening behind the scene?
A: As we know, Time, Tide and JS wait for none. And it's true. Over here it appears that JS engine is waiting but JS engine is not waiting over here. It has not occupied the call stack if that would have been the case our page may have got frozen. So JS engine is not waiting. So if it is not waiting then what it is doing behind the scene? Let's understand with below code snippet.

const p1 = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve('Promise resolved value by p1!!');
  }, 5000);
})

const p2 = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve('Promise resolved value by p2!!');
  }, 10000);
})

async function handlePromise() {
  console.log('Hi');
  debugger;
  const val = await p;
  console.log('Hello There!');
  debugger;
  console.log(val);

  const val2 = await p2;
  console.log('Hello There! 2');
  debugger;
  console.log(val2);
}
handlePromise(); 
// When this function is executed, it will go line by line as JS is synchronous single threaded language. Lets observe what is happening under call-stack. Above you can see we have set the break-points.

// call stack flow -> handlePromise() is pushed -> It will log `Hi` to console -> Next it sees we have await where promise is suppose to be resolved -> So will it wait for promise to resolve and block call stack? No -> thus handlePromise() execution get suspended and moved out of call stack -> So when JS sees await keyword it suspend the execution of function till promise is resolved -> So `p` will get resolved after 5 secs so handlePromise() will be pushed to call-stack again after 5 secs. -> But this time it will start executing from where it had left. -> Now it will log 'Hello There!' and 'Promise resolved value!!' -> then it will check whether `p2` is resolved or not -> It will find since `p2` will take 10 secs to resolve so the same above process will repeat -> execution will be suspended until promise is resolved.

// 📌 Thus JS is not waiting, call stack is not getting blocked.

// Moreover in above scenario what if p1 would be taking 10 secs and p2 5 secs -> even though p2 got resolved earlier but JS is synchronous single threaded language so it will first wait for p1 to be resolved and then will immediately execute all.

Real World example of async/await

async function handlePromise() {
  // fetch() => Response Object which as body as Readable stream => Response.json() is also a promise which when resolved => value
  const data = await fetch('https://api.github.com/users/alok722');
  const res = await data.json();
  console.log(res);
};
handlePromise()

Error Handling

While we were using normal Promise we were using .catch to handle error, now in async-await we would be using try-catch block to handle error.

async function handlePromise() {
  try {
    const data = await fetch('https://api.github.com/users/alok722');
    const res = await data.json();
    console.log(res);
  } catch (err) {
    console.log(err)
  }
};
handlePromise()

// In above whenever any error will occur the execution will move to catch block. One could try above with bad url which will result in error.

// Other way of handling error:
handlePromise().catch(err => console.log(err)); // this will work as handlePromise will return error promise in case of failure.

Async await vs Promise.then/.catch

What one should use? async-await is just a syntactic sugar around promise. Behind the scene async-await is just promise. So both are same, it's just async-await is new way of writing code. async-await solves few of the short-coming of Promise like Promise Chaining. async-await also increases the readability. So sort of it is always advisable to use async-await.

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