How async/await really work

How async/await really work

“An async function is a function declared with the async keyword, and the await keyword is permitted within it. The async and await keywords enable asynchronous, promise-based behavior to be written in a cleaner style, avoiding the need to explicitly configure promise chains.” - MDN


To simplify how async/await work, we’ll use the fetch function from the window object.

const getTodo = async () => {
  console.log('Log me first');
  const response = await fetch('');
  const todo = await response.json();


console.log('Log me second');

Let’s explain what’s happening here.

  • In line 1, we declare a function called getTodo.
  • Next, we invoke the getTodo and immediately create a new execution context. At this time, assume in 1ms, we are going to get the message Log me first.
  • In getTodo execution context, we define a variable called response. For now, it is undefined.

response is going to be the evaluated result of the fetch function on the right-hand side. So it will get a Promise object. In this object, we have two important properties: onFullfilled and value. The value is gonna be auto-filled in with whatever comes back from the web browser. The onFullfilled is an array of functions, this array will be filled when we use the then method of Promise. We hold this object just for now.


In this time, let’s look at the web browser. We see we are spinning up the background feature XHR request by running the fetch function with an URL.

XHR requests have done many things, one of them is to send a request to the server. It sends an HTTP message to the server. At 1ms, in the complete phase, it not complete yet.


On completion, we want to update the value property.

Take a note: we have not assigned the Promise object to any particular place. It’s just a position in memory.

So the value here is the value of the Promise object.


And now the powerful await keyword is going to throw us out of the getTodo execution context. Where we are going to encounter the next line of code.

  • At the next line, we declare a variable called todo. And at this time it is also undefined. We run the right-hand side and we met the await keyword. This also throws us out of the getTodo execution context to run the next line - console.log(‘Log me second’).

The important we want to do is set up an asynchronous task that takes a bunch of time to complete - That is a web browser feature task, for example, to speak to the internet and take about 1s to complete but we want able to continue running our synchronous code afterward. You can see to do it, we are stepping out of our function.

But it would be wonderful if we could step back into our function when we get the value from the request back as a response. Hopefully, it’s gonna be stored in the response variable. And we can continue running our code and log that data.

After log 'Log me second, we don’t have much stuff to do. But assume after 200ms, we get the value back from the request. That going to update the value property of the promise object that is being stored in memory and referred to its position.


At this point, we are going to trigger at that moment the continuation of our getTodo function. make the getTodo back on the call stack and come back in.

The await keyword was super powerful, it threw us straight out of the execution context that we never got to assign anything to the data. But it’s not a bad thing, we are hoping that data is going to be filled in with whatever the right-hand side evaluates to. And the value we get here is the response from the server.


And we assign this value to the response variable.


After that, the todo variable also gets the evaluated value of the response variable.


So that we can hit our next line - console.log(todo).



All works return a value or the response object into the promise object we stored in future data. Then we use the then method to trigger the callback function to receive the data. All of that is automated. The async function will handle that for us. The await still behaves similar to a yield keyword.

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