Why Are Lines in JS Executing Upside Down? Understanding Callbacks in Asynchronous Code

Discover why JavaScript can execute code in an unexpected order and learn how to fix such issues with asynchronous functions and callbacks.
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This video is based on the question https://stackoverflow.com/q/68251983/ asked by the user 'Hello Universe' ( https://stackoverflow.com/u/1887791/ ) and on the answer https://stackoverflow.com/a/68252183/ provided by the user 'mplungjan' ( https://stackoverflow.com/u/295783/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Why Are Lines in JS Executing Upside Down?

If you've ever found yourself scratching your head over why line 2 in your JavaScript code runs before line 1, you're not alone. It's a common confusion, especially when dealing with asynchronous operations. In this post, we’ll dive into why this happens and how you can effectively manage your code to avoid these pitfalls.

The Problem: Confusing Execution Order

Let’s start with a quick look at the issue:

You have a function that converts files into base64.

After calling this function, you expect to immediately access the base64 data.

Instead, you find that a later line of code runs before the one you thought would run first.

Here is a simplified example of your original code:

[[See Video to Reveal this Text or Code Snippet]]

In this code snippet, you expect console.log ('Line 1 \t' + fileToSend); to execute before console.log('\n Line 2 \t' + fileToSend);. But it does not—let's see why.

Why Is This Happening? An Introduction to Asynchronous JavaScript

The root of the confusion arises from how JavaScript handles asynchronous tasks.

Asynchronous Code Execution

File Reader: The FileReader API is used to read files asynchronously. When you call reader.readAsBinaryString(file), JavaScript initiates the read operation and moves on without waiting for it to complete.

Callbacks: When the file reading is done, it triggers the onload callback function. This function is called only after the file has been read and does not execute immediately.

Because of this non-blocking behavior, any code that comes after the asynchronous call (such as console.log('\n Line 2 \t' + fileToSend);) may execute before the callback does.

The Solution: Handling Callbacks Properly

To manage the execution order correctly, you need to ensure that any operation dependent on the asynchronous read is placed inside the callback function. Here’s the modified version of your code that resolves the issue:

[[See Video to Reveal this Text or Code Snippet]]

Key Takeaways

Understanding Callbacks

Define: A callback is a function that is passed into another function as an argument and is invoked after some operations are complete.

Code Structure

By placing dependent code in the callback, you ensure it runs only after the asynchronous operation completes.

Conclusion

Understanding how asynchronous JavaScript works is crucial for preventing execution order issues. By appropriately using callbacks, you can write cleaner and more effective code. The next time you find lines running in an unexpected order, remember this principle of asynchronous programming!

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