Asynchronous JavaScript

Learning goals

  • Understand why we need asynchronous javascript
  • Understand what happens behind the scenes when asynchronous javascript runs and the steps it goes through


  • Execution Call Stack A data structure for the interpreter to keep track of its place in a script that calls multiple functions. It is single threaded and LIFO (Last In First Out)
  • Web APIs Built in APIs that have a separate execution context from your code (They act as separate threads) examples include setTimeout and fetch
  • Callback Queue A data structure that holds functions returned from Browser APIs that are ready to go back on the stack. It is FIFO (First In First Out)
  • Event Loop Middleman between callback queue and call stack. Checks if call stack is empty and adds functions from the callback queue

Let’s talk more about JavaScript and it’s ability to be asynchronous. What does that mean, and how can we wrangle or take advantage of that fact?


Using sticky notes in this Jamboard, answer the following with your breakout group:

  • Everything you know about & questions you still have on asynchronous code
  • Everything you know about & questions you still have on fetch
  • Everything you know about & questions you still have on execution contexts & the call stack


When we say single-threaded, that could be read as doing one thing at a time, one-by-one until the code is done. When we say asynchronous, that could mean that multiple things are being done at different times. How can JavaScript do both? They seem to be in conflict with one another.

On Your Own

Take several minutes to read this article about how JavaScript is asynchronous and also single threaded.

Then in breakout groups, explore this site that gives a nice visualization of how asynchronous code runs in the browser. Note the call stack as well as the Web API the article discusses.

  • When running the code, note where the console.logs are moved. What about the event listener or setTimeout?
  • Where does the setTimeout move to before going back to the call stack? What do you think this callback queue is?
  • Click on the Click Me! button on the bottom left-hand corner. Discuss the order of events.


Notice that this article does not mention “promises”, which is a way to deal with asynchronous JavaScript that we have been using with fetch.

With Your Partner

Given the following code snippet:


  .then(response => response.json())
  .then(data => console.log(data))
  .catch(error => console.log(error));

  • How does fetch fit in with the code above and below it?
  • What order will the console logs show up in and why?
  • Using terms like the call stack, Web API, & callback queue, explain this is working behind the scenes.

Call Stack and Event Loop

How can JavaScript keep track of what code or function is running at any given time? How can we keep track or visualize how to expect the code to behave? JavaScript handles this using a few tools: the call stack, event loop, and a queue.

On Your Own

Let’s take a look at this video that has some great visualizations for the call stack, event loop, and queue. Pay close attention during the following points:

  • 4:15-6:00 (call stack with synchronous functions)
  • 6:40 (stack overflow example)
  • 7:20-8:50 (imagine if fetch were synchronous - we would have to wait for each request to complete before moving on)
  • 10:23-11:46 (asynchronous callbacks - setTimeout, what happens to the setTimout on the stack?)
  • 11:47-14:50 (intro to the event loop)

Please take notes and pay special attention to the vocab words at the top of this lesson as you watch

Check for Understanding

  • Review how the call stack works with synchronous events?
  • What is involved with asynchronous events such as setTimeout or fetch?
  • Describe the relationship the event loop has with the callback queue and call stack.

Additional Resources

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